With the current implementation $bottomN returns results in the order you would get by using a $sort stage with the same sort spec, but $topN does not. $topN currently returns results in the order opposite of the sort spec, e.g. {a: 1} currently returns the largest $a first whereas a $sort stage would return the smallest $a first.

Pending on going discussion of result ordering in the Exact Top N Accumulator project, we should update AccumulatorTopBottomN to reflect that decision and change all the tests.