Thank you for your suggestion. Since posting today , I found the white paper on Apple ProRes on the Apple site which provided the answer:
The Apple ProRes family also supports the 720p HD
format at its full width (1280 x 720). In addition to full-width HD formats, Apple ProRes
codecs support three different “partial-width” HD video formats used as the recording
resolutions in a number of popular HD camcorders: 1280 x 1080, 1440 x 1080, and
960 x 720. While Apple ProRes formats can encode image frames of any dimensions,
only the SD, partial-width HD, full-width HD, and 2K frame sizes are supported for
real-time editing in Final Cut Pro.
Thanks
Dana Thomas