This is a coincidence. The reason the 2021 Performance ended up at 499km originally is because 481km (299mi) was the 2020 range, and 299mi*~80.6/77.8 = 310 miles. Note that the LR AWD 2021 vehicles were nowhere near this value (much higher, though variable - separate topic).
It's a complete coincidence that the 2021 Performance ended up the same as the 2018/2019 vehicles initially (remember, NONE of the 2020 vehicles had 499km of range except for a brief period when they were brand new in late 2019, before Tesla updated the constant).
Basically, the 2021 Performance were assigned the same constant as the 2020 20" wheel version of the Performance, and were given a larger battery. So that ~299miles went to ~310 miles (ratio of new and old capacity). And then subsequently they updated your constant to give you the rated 315 miles (no change in capacity).
Pure coincidence; that 310 miles is quite different than the 310 miles on the 2018/2019 Performance, for a couple reasons. This just shows again that comparing miles degradation across model years (and even within the same model year if the software changed the constant, which does happen, but very rarely, and only at the very initial part of a model year as Tesla software catches up) has to be done extremely carefully. And personally I would separate out different packs so they could be compared to one another, not combine them.