Citation needed.
It makes no sense for Google to market the 4, 7, and 10 as a coherent, unified Nexus family with ads that feature all devices together and then have one of those devices follow a different naming scheme. AFAICT, the 4 referred to the screen size (which is better? to round up to 5 and have people angry with you because it's really 4.7, or to just floor it to 4 and be safe?), and that it was merely coincidence that it corresponded with the generation. Likewise with 5.
Google reset the Nexus branding when they introduced tablets into the mix. With the Nexus S and Galaxy Nexus, they were co-branding with Samsung's own product lines, but with last year's Nexus batch, they reset everything. The N10 isn't the "Galaxy Nexus Tab", the N4 isn't the "Optimus Nexus" or the "Nexus G", and the N7 didn't carry any naming from whatever Asus's brands were, and unless evidence to the contrary is presented, I believe that this reset was to produce a coherent, unified brand and naming scheme of "Nexus Inches".