I am doing Champion Mipha's Song on Master Mode and I am on the 'Defeat the ancient foes' one, meaning I have to kill some Guardian Skywatchers.
I've found that my Perfect Guard damage is changing compared to the other Guardian types. Normally your parried laser blast deals 500 damage, but there are many times when my damage is only 104 which is then quickly recovered from.
I have found that they have been swaying either from damage or wind, and if I am higher I will always miss with the reflected blast. However, when they are higher I am more likely to hit.
I originally thought that shield values were where the damage came in, but using a Steel Lizal Shield (35), Knight's Shield (40), Royal Shield (60), and Guardian Shield++ (42) I found they have not directly affected damage.
Why do I sometimes get the 500 damage, but other times it's only 104?
Best Answer
Firstly, as you may know, hitboxes in BotW are super tight.
Secondly, partial hitbox collision is a thing - the most common example of this is the "Bomb Impact Launch" (BIL). Depending on "how it hits", it can either cause minimal damage, and launch Link, or cause no damage, fail the launch, and Link gets hit by the explosion instead (ultimately causing more damage).
In the following clip, we can see that Link gets hit by the round bomb, knocking Link, but causing no damage, and then getting hit by the explosion instead. The rest of the successful BILs cause ~1 heart of damage.
Therefore, as you mentioned in the OP, the Guardian's swaying movements may have caused a "Partial Impact" on the hitbox of the guardian, causing less damage. Unfortunately, I have been unable to find more information as to the science of this at this time.