Keyframing Bug?

Trying to add a keyframe zoom and FCPX is doing something slightly odd. Instead of a straight line zoom between the two keyframes, it's doing a little shimmy. It's a nice shimmy but it's not being asked to do it. Any ideas please? Clip attached below...

Select the viewer and enable the Transform button.
You can now see the Motion Path between your two keyframes. If you Right click on the
white Keyframe indicators it should reveal a Linear/Curve option. Even though it's set to Linear you still get the 'smoothing' effect between keyframes.
Position the play head in the timeline between the two keyframes and, using the 'Add Keyframe'
button top left of the viewer, keep adding and deleting keyframes until the position of the clip stops moving. make sure you haven't created a keyframe and run the clip. It should now have a linear path. Phew!

Great, will give that a go. Thanks!

This doesn't make much sense, but it's a workaround that works.