The same happens with sheeps, cows, chicken...
Zombies appear even not to be influenced by this.
That effect just makes you hover for a certain amount of time.
The other amplifiers do the following:
0-127 : levitates
128 - 254 : changes the gravity (allows gliding etc.)
255 : hovering
Idk why it is, but it is pretty cool and handy for adventure maps.
ps: other potions sometimes have similar functions.
This happens to all mobs that have a "glowing" effect