We're talking about how you have a low expectations and get pleasantly surprised. Thus I put the B on the movie.
More on the matter, in the same thread you rewarded Expendables2 with C. See… That movie was a clear A to me and can't wait for the next sequel. And that one didn't impress you either nor you remember anything from it I suppose.
Stash House and 6bullets you've also mentioned, sorry, didn't watch those (and probably never will) so can't say anything about them.

I simply do not share the common feel that horrors and action should never get A or B and that A or B are reserved only for drama. So if I enjoyed a movie much, why would I just lower the rating just because it's this and that genre?