audio compression file size, doesn't work

I change the compression kbps with the audio impot setting, and the Inspector shows me the new file size. but when I build assetbundle use it, the file size is not same as the compression size in the Inspector.

see the pic, whenever I change the comression kbps, the assetbundle is always 1.8M. But the compression shows the new file size 0.6MB. Why it not appears to the assetbundle size[56511-qq截图20151020110903.jpg|56511]? Is this a bug to unity?

my unity version is 4.6.5

The same problem which bothers me a lot.
Anyone has any ideas?