Whenever these arguments are used not by a player, the test fails.

How to recreate:

Use /testfor @a[y=0,y=10].
This should detect every player higher than y=0 but below y=10.
In my test this worked.

Now use the same command in a repeating command block set to always active and set commandBlockOutput to true so you can see how the command gets performed.

Even though it's the same command, it doesn't work.

/execute on a player DOES work.

/execute on an entity DOESN'T work.

Strange, huh?