I am evaluating Cineform HD codec for use as our digital master format and I am encountering some interesting behaviors. When I encode a file directly from within Quicktime Player (Mac), I get a resulting file that is 100Mbps, but when I use Episode Pro (which uses the Quicktime encoder) I get a file that is 55Mbps. Likewise, when I use Compressor, I get a file that is yet another bitrate: ~75mbps. The encoding settings are identical in each application.

Since the Cineform interface does not give an option for bitrate, I'm assuming that the bitrate is automatically selected. The source content I'm encoding is 1280x720 and I'm outputting to 1280x720 @ 23.976fps. Why would the bitrate be different when using different encoders, even though the source material is the same?

They should not be doing that. If you send a 1 second clip from each to support we can determine what each application is doing and why they is a difference.