OK, I'll bite.
Canon patented a radically new sensor type a few years ago that would be able to do 1) global shutter, 2) nearly double dynamic range, 3) electronic neutral density filter.
Maybe that sensor is either low resolution, or fiendishly expensive, so they give you a choice between the current sensor or the new one? (I don't follow other brands; is the R5's sensor no longer cutting edge or near to it? When it came out I recall it was near the top for DR and noise but that was several years ago.)
Alternately, maybe the focus-follows-your-eye sensor makes it bulkier or more expensive so again they're offering both models?
I love my R5 so much that I can't see a way to improve specs, really. I don't need more MP or less noise or more ISO or better colors or longer movie shooting times or faster burst speed etc. etc. etc. (I'd like more slo-mo and timelapse options built-in, for instance, but that might be as simple as a software upgrade.)
But I'd totally consider an upgrade to my R5 if the improvements were something like focus-point-follows-photographers-eye, or say some of the features I describe above whether or not they're thanks to a new sensor.