If you look at coin.dance, it looks like the coinbase transactions text start with consecutive letters. For example, block 478529 starts with "AM" and 478530 starts with "BM" and 478531 starts with "CM" and so on. This continues with DM, EM, ... all the way to ZM despite being mined by lots of different miners.
This looks coordinated in some way. Why is this? Can't the coinbase text be whatever the miner wants?