CBR files of identical length exporting as different file sizes.

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I am exporting some videos of identical length as H.265 mp4s.  I choose hardware encoding and a CBR.  I use this same preset for all videos but the final files end up as different file sizes.  Shouldn't they be identical since I am using a CBR?  It seems they are only ending up as identical final file sizes when I use software encoding but this takes over 10x as long.  Thoughts?