If using AVCHD media and you end up with media to render, it gets rendered as a MPEG2 file at 25Mbps! (AVCHD is MPEG4, so I would have expected the render to follow the same video format). OK the bit rate is similar but.....
(Yes I was using an AVCHD sequence)
If using AVC intra100 (I made one by converting a file in AME), in a AVC intra 100 sequence, It does the same thing. That is rendered video is MPEG2 at 25 Mbps! That kind of destroys the use of 100Mbps AVC. And AVC intra 100 is MPEG4 based.
I guess that if you want to avoid exporting a completed programme based on a mixture of original high bit rate footage and footage rendered at 25Mbps, you have to make sure that "Use Previews" is not ticked in the Export settings, so as to make it re-render all the media.
Beware all those of you exporting for broadcast use.
Now you know why we say not to use preview for export.
Live and learn!