More like it's just a name to separated launches of a new phone. For the history, both series has their own unique specification, features support & differences. As like @ Edster89 said more expensive, more specialised for the series had followed by no of the series given.
But still both are mid range phone, it's still have lack specifications nor features compared to the flagship. Just in some of it got higher than maybe only 1 or 2 specifications like battery, camera or screen.
You can assume it's just a name they given for the series name. As like we know now there is Not S11 to continue S series. They change to S20 which is because of the year now is 2020.
It's hard to compare between this 2 series since there is no new M series now. Some said that before this M series known as J series. Might A series new launched that added 1 no is the continued of M series. Hahaha😅
