I don't get it either, but I think FCP must drop into another mode for playback or whatever. I don't have HDV, and really, don't want HDV as I find it way too limiting, especially with such issues as this. DV wasn't just a revolution because of quality, but it was easy, the files were small, simple connections, monitor while editing, etc. etc. HDV doesn't have anywhere near as much going for it as DV does in the usability stakes.
Graeme
[
www.nattress.com] - Plugins for FCP-X