So
4600 > 480/580
6800 > 660/680
8800 > 780/780Ti/980/980Ti
The numbers don't mean anything and were always confusing. What's faster GTX580 or GeForce 5800Ultra? The only reason I know is because I remember what came first. To someone who doesn't know videocards, you can't say that 580 beating 5800 Ultra is a logical naming convention.
GPU naming has never been logical, that's why you end up with five different GT 640s; three different dies from two different architectures and a mix of DDR3 and GDDR5.
5800Ultra and GTX 580 are a little different though, since those came out close enough to a decade apart. They aren't likely to be directly compared. With the architecture naming scheme, even if your average guy gets told that M stands for Maxwell and P for Pascal, that doesn't mean anything to them. The OEM manufacturers need something that they can at a glance show is better on a spec sheet; that's why we have the OEM HD8xxx where the extent of the changes was a single number to differentiate it from the previous generation.