While keeping in mind that Yami does this on her freetime and that I don't have the slighest clue of the time constraint moving players up and down represents, the fairest option would be to add another category of 'general' statistics but for each knockout stage respectively. That way, you can see how DivB/C promoted players do when they play against an higher opposition without having their stats being mixed with their previous matches in the lower division.
I am not a DivA player and yet I totaly understand why some don't find the current situation 'fair'. It's not a big deal but since statistics are being made by people who took a lot of time working on them, I immagine they would want them to deliver the most accurate and coherent picture possible right? Effectively however, it probably represents a huge amount of work just to keep that overall coherence.
No matter what, thanks for your amazing work as per usual Yami!