I think the processing extent is a rectangle. If you give it a mask with a non-rectangular feature, it will set processing extent using the extreme North-South-East-West coordinates of the envelope that contains the feature. When I've gotten results where the processing extent was somehow set smaller than I wanted, the results were always withing a rectangular "envelope" with N-S-E-W limits. I think to eliminate the data outside, you actually have to use the mask to clip the data or extract by mask. Depending on what you are doing, you may still need to deal with results outside of the data that is left over after the clip or extract.