It's because of the way unbalancing works in the game. If you unbalance someone (say, by parrying them while they're OOS or you're in revenge), they will fall down. However, if you strike them too soon while they're being unbalanced, they'll stay standing. This is most likely because the devs didn't want to lock someone into that animation of falling to the ground if they're being already attacked, as that would mean you could basically land a full combo for free against them.
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