I find it confusing to be honest.
3090 has 19500 MHz memory but I added 250mhz and Afterburner reports 10,000Mhz? I guess it's DDR though making it 2GHZ?
And the Nvidia boost thing means the core jumps around a bit anyway.
If afterburner reports 10,000 just divide by 2 to 5000mhz.
Problem is there are so many ways of writing memory speeds, your 3090 has a stock speed of 19500 MHz which you have to divide by 4 to get 4875 mhz.