The problem is what is reported to the pool.

kano knows if it is an S9, because the miner tells the pool what it is, same with the A6+

He can only compare the numbers that he has access to, he can't (I think) separate out the A6 from the A721. Most likely because they are running the same software. Also the S7 (and older) does not announce itself like the S9s do.