Transitions mess up audio sync

I'm using iMovie 08, and I'm putting together a simple, short movie. There are 3 clips which I want to add transitions between. I've added Fade Through Black 1.5 second transitions, no problem. But then when I play the movie back, the audio becomes more out-of-sync after each transition. It is 1.5 seconds out of sync after the first transition, then 3 seconds out of sync after the second transition.

It appears that the transitions affect the video playback, but the audio is playing continuously, as if the transitions weren't even there.

For example:
Video Clip One: 30 Seconds
Transition One: 1.5 Seconds
Video Clip Two: 30 Seconds
Transition Two: 1.5 Seconds
Video Clip Three: 30 Seconds
Total Video: 93 Seconds

However:
Audio Clip One: 30 Seconds
Audio Clip Two: 30 Seconds
Audio Clip Three: 30 Seconds
Total Audio: 90 Seconds

Is there something I'm missing here?