vertical velocity has a bug

I build this vizzy program,and I found something strange.The vertical velocities are not same,and they have serious difference.
This is my vizzy:

When i entered the game,they were about same:

That seemed every thing is normal,BUT,after i drove my craft for a while,something different happened.

The third one began to change from another two.I don't know why,and this is really annoying.I hope that someone can tell me the reason and fix it.