It's an unpopular opinion but I think he should have got the MoS award coz apart from the 14 wickets he took in Mumbai, he did well on a dead Kanpur track where most visiting spinners would have done (very) badly and also NZ helped saved the game.LOL so you do have intentions!
What you say is all very true and we'll never really know if Ajaz actually bowled better or not than Ashwin or Siraj.
However, apart from the the raw statistical evidence in the match, there are 2 vital pieces to the script.
1. MAYANK scored his runs in home conditions, against an unusually lacklustre NZ attack.
2. AJAZ got his haul of wickets away from home, against an extremely strong Indian batting lineup.
For the second test, the pitch had good assistance for both pacers and spinners and if you remove Mayank's century, India was going to be in a terrible position(might have even lost who knows). On the other hand Ajaz's 10fer didn't leave much of an impact on the game even though it was a great performance.