File Sizes

This has me really puzzled.

I have a Rexing V1P Pro and was curious about how long I could use a 256GB card in single and dual camera modes before the system started overwriting. The Rexing manual gives some information but not enough to answer all my questions so I ran some tests. I testing almost all modes, but the only ones I don't understand are the FHD-P30 so that is what I have listed.

Single Camera mode on 256GB card - FHD (1920x1080P30 - about 37.1 hours

Dual Camera mode on 256GB card - FHD (1920x1080P30 for both) - about 27.5 hours

My timing tests correspond almost exactly with the figures in the Rexing manual, so I assume they are correct, but the thing that bothers me is the comparison between the FHP-P30 setting for single camera and for dual camera modes. In single camera mode a FHD-P30 3 minute video takes 344MB but in dual camera mode each FHD-P30 3 minute video takes 232MB.

Why would the same recording resolution take up more space in single camera mode than in dual camera mode? Can anyone tell me?