This is how the rewards work:
Because they have no rewards for the “masks but not full clear” situation, if you miss something by accident, you drop all the way back to the 430 level. It is very easy when doing Stormwind to miss an objective - usually one of the 8 cages in Trade District. It happens so often that there are regular reports here of a bug where it didn’t count one because the player couldn’t possibly have missed one. But I know I’ve done it once.
And the way the diminishing returns works on rewards, if you do it twice, the next time it drops you down a tier and you only get a 420.