I think that's a different question; i.e. the ratios of 756x504 and 720x480 are both 1.5:1 (or, as we say in the camera world)

IMO the most likely factor for camera makers (and/or sensor suppliers[0]) is 'how much resolution can one pack into a sensor for marketing purposes while keeping yield margin in check for profit'. Megapixels instead of Megahertz...

[0] - Maybe different back then but the current state is that even the 'big names' will get their sensors from 'competing' camera makers...