Yes, 15 to 21 is not really a small number.
Donīt forget the process of verification of victories is not closed, also not for Mannock.
Additional different sources give different numbers of verified victories for the same pilot. I guess you were using
ATT.
Donīt forget 20+ would bring him a Pour le Mérite on the German side as well! Al Lowe would give his last shirt to achieve a similar number for Bishop!
And: With the application of more rigid rules pilots would also get a VC for a lower victory number! Donīt you think so? It is not fair to apply the harder rules only to Mannock without doing so for Tempest, Clayson or Burdon as well.