Creating motion tween query

Hi,

I am creating a motion tween on the timeline with a number of different frames. The aim of this is to create a fade in effect which is working perfectly. The problem is when I create a tween on the layer that just contains three buttons it knocks the black line off the left side of the keyframe to the right. This is shown below.

Any ideas?

I have only created the tween between frame 20 and 39 on each of them.