AVCHD: bitrate difference of file vs. disc for same project?

[dalemccl]

I use PowerDVD 8 Ultra to view video on my PC. It can display the bitrate as it plays. The native 1920x1080 AVCHD clips from my Sony SR-11 show a bitrate that varies between 14k and 16k, usually toward the higher end of that range, and occasionally exceeding it a little.

After editing clips with Pro X2 (with the November patch and the November DirectX update), I normally create an AVCHD file using "Share- Create Video File - AVCHD", with resolution of 1920x1080, max bitrate of 18k, UFF, quality slider at 100%. I don't use SmartRender. The resulting video file, when played back with PowerDVd shows that the bitrate generally varies from 10k to 12k. This is significantly lower than the native clips' rate of 14k-16k.

But if I use the exact same project and instead of creating a file, create a hybrid DVD+R disc directly from the project by doing a "Share-Create Disc-AVCHD", with settings of variable bitrate max of 18k, quality slider at 100%, 1920x1080, UFF, the playback of the disc shows that the bitrate varies between 14k and 16k, usually at the upper end of that range and occasionally exceeding a little. In other words, at the same bitrate range as the raw clips.

So it appears to me that Pro X2 creates a lower bitrate when generating an AVCHD file as compared to when burning a AVCHD hybrid disc directly from the project. This is with all settings being the same, whether creating a file or a disc.

Any ideas why this would happen?