Are M sized sockets the same as MM metric sockets?

[jbrianfoto]

Hi all. I am looking at the cam sprocket on my om603 engine. It is a 12 point bolt. The FSM says it is an M11 sized 12 point. I went to my tool box, pulled out my metric 12 point sockets send found that only a 13mm socket fits, though it is a little bit loose. So, are they different, M vs mm?

Trying to decide if I need another set of M sized sockets of if my mm ones are ok.

Thanks!