Yeah I contemplate that all the time. Best I can tell it's not a direct correlation. For example, a pickup with one coil at 5K and the other coil at 10K wouldn't have "half the hum-cancelling" effect as one with two 5k coils. (i.e. twice the hum) In other words, the fact that two RW/RP coils exist is a bigger part of cancelling hum than the actual resistance match. To what effect I'll never know, (and I don't care) But I'm sure it's a variable and not constant. In other words, the majority of noise is cancelled once you get to within a certain range, and it's less apparent after that.
As for me, my gear is pretty good, and I have a lot of guitars with single coils too. So a slightly noisier humbucker is like getting a free box of chocolates, and then you notice one's missing. I'm just glad it's cancelling hum. I've had a couple guitars with variable coil bleeds so I could dial in how much of "coil 2" I wanted. Now, they were audio taper pots so it's impossible to say at which point this occurred, but the phenomenon was that I could get to a certain point where it still sounded very much like single coil mode, but with a fair amount of hum cancelling too. Sort of a "best of both worlds" scenario. I guess I could've measured the output of each coil at that point, but that was before you posted, and now the coil bleed pots are no more! Plus that wouldn't directly answer this scenario because I was removing output, not coil windings.