why the mecanim state time doesnt start from zero

first,you may have noticed that, the state clip starts a little offset from zero.Why was that?

secondly,the animation clip's length can't match with the length of the clip in animation state.You can see the picture I attach.In fact the animation clip last for 6 frames,but 9 frames in mecanim state.Why was that?

There will be some mistakes,when Im using codes to control the time precisely.For example,there is 16frames for me to input a key to change into fly state,when the jump state start.Then I set the end of the transition to about 16frames in the preview.If I input the fly key at the very end of the transition,say 15frame, there will be a fly trigger,but no fly animation,which means the animation time is not the same as the game time?

atomic is surely unchecked.

Is that a unity bug? hope someone can help me ,thank you.

 

I have found out that.It's 24fps in my original animation clip;but 60fps in my mecanim preview;and what's strange enough,the timeline in the mecanim preview is abuout 30fps,with a little offset.

That makes me confuse to set up time precisely.What's the purpose of such setting?different fps at different part.

 

I figure it out that the animation clip is produced in 24fps,but the game engine is running in 60fps. So the mecanim is running in 60fps,which is reasonable.
But why the preview timeline is in 30fps?