Keep in mind that Canon still cameras use DIGIC processors, but the video cameras use DIGIC DV processors. They are not the same.
I believe the hardware encoder in both processors is the same however, but the difference is in how the processors read and process the output from the sensors. The DIGIC processors are optimized for stills, with video capability as an extra, but they are not particularly good at that. It is a compromise, with video being compromised in favor of stills. The DV processors on the other hand are optimized for video. The image that is generated by the processor goes through the same H.264 encoder, so if the C100 provides high quality images while something like the 5D does not, the difference is in how the processor deals with the raw data, not Canon's implementation of the H.264 encoder.