Windows Movie Maker makes timing bewtween photos and music (audio and video) slightly off sync when saved to wma or mp4 file. any fix for this? why?

Using WMM on Windows 7 x64 and spend a long time fine tuning the timing on slideshows only to notice that an epic transition synced exactly to the audio/music gets shifted by about a half a second or more when the file is finally finished and saved to a wma file. I convert it to an mp4 file after that for stability during playback and still the timing is slightly off (which is really frustrating considering there's no way to share a true copy of your creations other than in the low resolution, in program, preview version. Why is this? Anybody found a solution to this or idea for a fix? 

Thanks for reading and help!