Why some animation curves change when click preview button on the animation window?

I import an fbx file containing an animation. At first the animation curves are like this:

Notice that Root T.y = 0.99905,But after I click the Preview button on the upper left corner,Root T.y changes to 0.87057

I find that the value is different when I preview this same animation on a different model. Maybe the value is affected by the model and I want to know how model affects this value.

Probably this value is stored in the first frame of the animation you are trying to preview.

I’ve found the answer. see Mecanim humanoids | Unity Blog

If an animation is imported as humanoid rig, curves in the animation window represent transforms in muscle space. And Unity will multipy curve value by avatar’s humanScale to get actual transforms of joints. So when you select different game objects to preview animation, if they use different avatars, the curve will change accordingly.