I just realised that all things aren't created equal when it comes to aspect ratio. We seem to use the term 16:9 for all widescreen video, 1080 HD would seem to be exactly 16:9.

For example 1920 / 1080 = 1.77777 (HD)
16/9 = 1.77777
1049/576 = 1.8211 (PAL Widescreen)

The differences are pretty minor however when changing project properties it does cause things like event pan/crop to shift enough to cause problems.
Bob.