On a side note, Falcon (and some other (most?) characters) have a reverse grab box on their running grab.
i think there's two things at work here:
1. the hitboxes themselves (obviously):
it feels like running grabs have more reverse grab range than running JC grabs. sheik's an obvious example.
falcon's grab hitbox reaches really deceptively far behind him, which is why some falcons (i think) will go for shieldgrabs that wouldn't work with anyone else whenever the opponent is like.... RIGHT on top of you. it wouldn't work with like, marth, fox, or falco, for example.
btw, iirc, mewtwo and falcon have the biggest reverse grab ranges in the game
2. hitbox interpolation:
hitboxes sorta linger in place from frame-to-frame. so for example, if you do a retreating aerial, it will retain some additional range from the hitbox on the previous frame (or something like that) and it will effectively have more range than a really advancing aerial..... so when applied to running grabs, the hitbox will linger BEHIND you, causing you to have a longer reverse grab hitbox.