Unexprected results from compression of bland scene

[cd]

Hey,

I was carrying out an experiment to see how effective HEVC is at compressing video sequences of bland background areas such as white walls.

I captured 1000 uncompressed frames at 1920x1080 and created two seperate YUV files. One of all the raw frames and the other was 1000 copies of the 1st frame.

I then compressed each YUV using the x265 encoder but the results were not what I expected. The final file size for the 1000 different frames was 421 kB however the 1000 repeated frames was 715 kB.

I would have expected these numbers to have been the other way around, can anyone explain what is going on here?

Thanks.