The feat Step Up triggers only versus a 5-ft. Step

Following the link, you'll see that the 5-ft. step is a specific action. Although other actions allow movement of only 5 ft. these don't meet the feat Step Up's conditions as moving only 5 ft. is different from taking a 5-ft. step.

The game provides no rationale

It's unclear in the rules how the character can differentiate between a 5 ft. step and, for example, taking the Withdraw action and only taking 5 ft. of movement, but somehow the character can. The game abstracts much of the character's ability to read the battlefield. If a rationale is necessary to maintain immersion, just say 5-ft. step, because it behaves differently mechanically anyway, obviously looks different to the character, and be done.

Following Step appears, to me, to pretty clearly indicate that it modifies Step Up. It's not an either/or proposition - to use Following Step, you must be using Step Up. So, reading them together, Following Step allows you to take a ten foot "5-foot step" which doesn't provoke AoOs and also doesn't subtract from your next turn's options (it still costs your immediate action). While you could theoretically use Step Up without the benefits of Following Step, since feats are optional unless otherwise stated, there's no reason to: Following Step kicks in as part of using Step Up, which means it applies to the 5-foot step granted by that feat.

The following rules citations were stolen from Hey I Can Chan's answer, bold emphasis mine:

The feat Step Up has as its benefit:

Whenever an adjacent foe attempts to take a 5-foot step away from you, you may also make a 5-foot step as an immediate action so long as you end up adjacent to the foe that triggered this ability. If you take this step, you cannot take a 5-foot step during your next turn. If you take an action to move during your next turn, subtract 5 feet from your total movement.

The feat Following Step has as its benefit:

When using the Step Up feat to follow an adjacent foe, you may move up to 10 feet. You may still take a 5-foot step during your next turn, and any movement you make using this feat does not subtract any distance from your movement during your next turn.

Closing note: I know, a 10-foot "5-foot step" doesn't exactly make sense. A 5-foot step is a defined game term, though, which means it works well as a shortcut - it's a quick way to say you can move 5 ft without provoking AoOs. Weirdness like that happens when one rule modifies another - such as allowing a 5-foot step to reach 10 ft.