Synchronize Clips command failing in a simple situation: what am I doing wrong?

I have a 38-minute XDCAM EX clip (brought into FCP X via XDCAM Transfer) and a matching H4n audio clip. I selected both in FCP X's Event Browser, and chose "Synchronize Clips" from the Clip menu.

The result is a Compound Clip that plays both the camera audio and Zoom H4n audio--but out of sync by about two seconds.

I know I can 'step into' the Compound Clip and manually sync the video and H4n audio, but what went wrong in the first place?

The camera audio is at a low level, but it's pretty clear, and I know PluralEyes wouldn't have a problem with this. (And yes, I know I can run PluralEyes in FCP X).

But I'm wondering, did I do something wrong or neglect some detail? I thought the synchronization in FCP X was supposed to be pretty solid. What's happening?