DVDSP changing the length of audio files

This is a problem that is driving me crazy. I can't work out what is happening to my audio files when I import them into an NTSC DVDSP project.

I have a PAL and NTSC version of the same project, using two audio tracks, 5.1 and stereo, both AC3 files created by Compressor. When I import them into the PAL project the asset length is correct 01:49:58:02. As soon as I click the NTSC button to change the project settings, the audio asset length is suddenly shorter 01:49:51:15.

When dragged into the timeline, the audio is too short and gets progressively more and more out of sync. The same thing is happening with AIFF files.

The audio was originally edited in Final Cut then exported using Compressor. The only workaround I have discovered is to export the audio out of an NTSC FCP timeline then export to compressor. This worked fine for the stereo track, but it takes quite a bit of time to re-export all the 5.1 tracks then convert to AC3.

I have also tried exporting AIFFs out of FCP then converting to AC3 (using Compressor), but hasn't helped.

Any ideas on what on earth is happening? Shouldn't the audio be exactly the same for PAL and NTSC projects? I have converted projects in the past without this problem.

Thanks in advance for any light you can spread on the subject.