Bug in after effects with key frames?

I've noticed this before, but this time thought I'd post.

I've got 4 keyframes. All easy ease. I want the two middle keyframes to be adjusted so that the camera slowly pans out (a little slower than the first two keyframes). This on a Null object camera rig.

So I adjust the Z position. This has the desired effect, but then I start getting movement on the Y postion. No other keyframes exist. I try deleting the third keyframe and pasting the 2nd keyframe in its place and then I start getting some weird movements on the Z position - the Z position shouldn't move at all as i've just pasted the same value as the 2nd keyframe so between the 2nd and 3rd keyframe it should be still right?

Olly Lawer