For applications submitted in the US, it's the date it is acknowledged by Sheffield... because the biometrics are taken several days before you actually submit your documents. They can't start processing anything if you haven't sent them your application yet.
However, for most other countries, it's counted from the biometrics date because you submit your application and documents AT THE SAME TIME as biometrics, so they can start processing it right away.
So, the reason UKVI say it's from the biometrics date is because for most people, it IS from the biometrics date. It's the US applications that are the anomaly here.