/entitydata downward motion glitch while going east.

When I was making a villager controller using /execute detect and /entitydata, whenever the villager was moving east, it wouldn't go down if i set the Y motion downwards. I went through the command, and copy-pasted it to go west. Guess what, it worked.

What I expected to happen was:
For the villager to move east and down.

What actually happened was:
The villager had no downward motion when I set the motion down.

Steps to reproduce:
1.Put this command on a 20-tick per second clock: "/entitydata @e[type=Villager]

{Motion:[-0.2,-0.2,0.0]}

"
2. Summon a villager in the air.
3. Watch it go east but not down.
4. Make the command go west and watch the villager move west AND down.