How to Explain Huge Disparity in Bitrates?

[MikeInThe901]

I capture VHS & camcorder video from a ClearClick upscaler using H.264 & AAC encoding to MP4 files. I get wildly different Total Bitrate numbers between files and I can't figure out why, after zero changes to the settings in between. Within two dozen files they range from 2666kbps to 15173kbps. It can't be content-based can it? What else controls that attribute? Thanks!