This is a micro design decision I made when I was possibly drunk (but it's still possibly good, hard to tell).
What is happening is that I've coded in a small extension of the perfect guard window's special effects in case you succeed with a perfect guard. So, while a perfect guard only triggers within 0.2 seconds after lifting the shield, attacks hitting you will receive any potential debuff effect for another ~0.3 seconds.
The reason why I added this way back was because back then, Perfect Stance (the perfect guard bonus) could not be used to cancel into shielding. So basically, I felt that in scenarios where two attacks (say lantern flames) came really close together, the player would deserve to perfect guard both projectiles if the first one was done correctly.
Now, though, since you actually can tap the shield button to "double shield" after a PG and that way receive two proper perfect guards, this system might be redundant.
(if you don't get what I mean with double shield: normally you can't spam the shield button because your character will end up lifting and lowering the shield, but after a perfect guard all shield animations become shield cancelable for a little while, allowing you to instantly re-activate the startup, which is what gives you perfect guards)