Robin Hood is an iconic scout: moving through the wilderness unseen and dropping arrows through the hapless tax collectors' apple from a hundred paces. He can appear out of the brush, attack, and fade back without the opponent having a chance to reply. Usually scouts are part of a larger group, whether a band of merry men, as advance guard for a squadron of soldiers, or checking the trail ahead for a band of adventurers.
A ranger, on the other hand, is more like Aragorn from Lord of the Rings. He can smell danger, track prey for miles over rocks, or calm a horse with a whisper. A woodsman, usually a loner (though sometimes with an animal companion), a ranger is often at home with a campfire, the stars, and a good knife. His skill set is very similar to a scout's, but with a basis more in nature than in proficiency with stealth or weapons.
XGtE is optional
It's important to remember that most of XGtE's content comprises optional rules, and this particular section on character generation is no exception. In fact, the opening on page 61 explicitly states:
IDEAS, NOT RULES
Even though these pages are full of tables and die rolls, they don’t make up a rules system — in fact, the opposite is true. You can use as much or as little of this material as you desire, and you can make decisions in any order you want.
Stretching that a little, we could infer it to mean that despite what might be implied by the options presented here, none of it should be taken as informing the actual rules of the game. It definitely doesn't help that this section of Xanathar's does contradict the racial history given for tieflings in their own description in the PHB. But if we want to take the content as presented and try and make it work, we could consider the variability of genetic expression.
All cambions are half-fiends (but not all half-fiends are cambions)
Simply put, though a cambion is a half-fiend and is always the result of union between a fiend and a humanoid, the union of a fiend and a humanoid does not always result in a cambion. Sometimes, when a devil and a humanoid procreate, perhaps the result looks more like a tiefling.
To draw a crude analogy to the real world, children of mixed race parents can vary wildly between strongly expressing the racial characteristics of one or the other parent or appearing somewhere in between - depending on exactly what random bits of genetics get passed on and in which combination. It's not even unusual for children from the same parents to appear to be completely different races!
Obviously the genetics of a fantasy world are much more complicated than ours could conceivably be (especially when you allow for magical factors). It is not much of a stretch to imagine that the children of a devil/humanoid pairing may sometimes express much more of their fiendish heritage (producing a cambion) and sometimes less (producing a tiefling).
What's the difference?
The general difference between a tiefling and a cambion is that a cambion is a very strong expression of fiendish ancestry that perhaps you can only get with one directly fiendish parent, but a tiefling is a much lesser expression of such ancestry which can also occur in descendants far removed from the original fiendish influence.
Best Answer
The Evidence: Hill Giant Height Change = Correction from 2e to 3.Xe
TL:DR - An Ogre Isn't A True Giant
If you just looked at the two monsters standing side by side, the Hill Giant would be slightly taller and a lot stockier and bulkier in build.
Based on the progression of D&D editions, the change in height appears to be a correction to the bloat/choices in 2e. Hill Giants have always been a much tougher opponent than Ogres(this goes back to OD&D: an Ogre was Monster Table 4 Monster, a Giant was a Monster Table 6 Monster). This theme of comparative power (Lore?) remains consistent with the stats in 3.5e.
While Ogres may be of the Giant Class or Giant Type (usage varies with edition) Ogres are not True Giants. That status is reserved for: Hill, Stone, Cloud, Fire, Frost, and Storm Giants. The distinction goes back to OD&D/Greyhawk when Storm Giants were introduced, and the term "giant class" was introduced. Hill Giants were always the least powerful Giants(Monsters and Treasure).
An early mechanical distinction in why "Giant Class" matters had to do with Dwarves having an advantage fighting against them(ref Monsters and Treasure): Dwarves did +1 damage (Greyhawk)and took half damage versus giant and other large clumsy creatures like Ogres(Monsters and Treasure).
Also certain weapons were "Giant Slaying" -- the special damage bonus applied to giant class monsters. (Refs: Monsters and Treasure, Greyhawk).
Bugbears were added to "Giant Class" (Greyhawk). Gnolls and some Orc kin (1e AD&D). (Aside: Rangers got a damage bonus per level versus Giant class creatures. As with Dwarven advantages, there were some mechanical benefits based on that classification ...)
I mention the above to point out that the "Giant Type" looks like the red herring that prompted your question. A better comparison is between your 3.5 Hill Giant and other True Giants to see if the power gradient lines up correctly.
Supporting points:(Stat format = HD/DMG/Height). The increased Giant Heights stand out in 2e. Compare that to the 3.5e Monster Manual: is the trend consistent?
Bottom Line: this difference is consistent with D&D lore from its inception. The 3.5e MM(SRD) shows a progression and comparative power like what's always been in the game.