Since Minecraft PVP basically sucks the chrome of a '57 Chevy Bumper, fights in Minecraft are sometimes stacked against somebody else, but not for the defeated players fault:
This includes, but not limited to...
- A player being hit far back enough, so where the enemy can keep hitting them, even without the enemy receiving any hits that the attacked player is trying to give them.
- Hitboxes being larger when under somebody, which is extremely random even for Minecraft.
- Ice PVP having nothing but chance involved, since movement hits are 99% of the time undetected by the hitbox.
The most common, and probably most relevant to your situation, is the Hitbox one. Yes, if you're below a player that's on high ground, your range will be farther than their's. But I believe that is the only increase that you get in sword/axe range in Minecraft, and I don't believe there are any decreases.