Caporegime
There is no industry standard formula. They decide a number ahead of time and back into it. The AMD calc literally has THREE floating variables that can change the TDP itself. You're also conflating what board manufacturers are doing with what's strictly a published spec by the chip vendor themselves.
Did you notice that the HSF resistance change is the only difference in how they got the 3600/3600x TDP values? That's not any kind of a standard. That's just coming up with a number ahead of time and adjusting the variables to back into it.
Here's a simple question. If put the same heatsink on a 3600 and 3600x, would they both consume the same power?
Depends on the heatsink.