Y, B and M fares earn a 50% bonus for miles and MQMs, along with paid first/business class. Your mileage statement page shows the fare paid and the fare flown. Since you paid for a M fare and flew in coach on that M fare, it will show M/M.
H, Q, K, L, U, T, and E revenue fares do not earn this 50% bonus, and you will receive 500 miles or the distance flown, whichever is greater.