This doesn't make sense to me. For all intents and purposes, the M position combines those of the AM and the DM, depending on the flow of the game and the strategic/tactical plans. While it makes some sense that a player might be proficient as an AM and a DM (but not a M) on different sides (such as a DML, AMR proficient player), if they are on the same side (L, C, or R), the M proficiency should come automatically, IMHO.