Yeah i was confused by that at first as i think a lot of people are. The MPC 2000/xl will give you the option (in the mixer mode > indiv. screen where you assign channels to pads) of having either 1, 2, 3, 4 or 1/2, 3/4 etc. based on whether or not the sample you're assigning to that out is stereo or mono. So you have to pay attention to which of your sounds are stereo and which are mono when assigning sounds to your 8-outs board.
Lampdog is indeed spot on, i just remember going through this exact issue and having this realization (about the MPC grouping outs based on stereo/mono) helped a lot.
I try to keep in mind the available channels on my mixer and only use stereo if its absolutely necessary to keep the sample sounding right. I've found the mix sounds cleaner if you use more mono tracks, and if there's only a slight difference between a sample in stereo when in mono then you might as well just convert it to mono. Once its in the mix you wont be able to hear that tiny improvement that stereo gave when you listened to the sample alone. Of course if there's stereo movement then you should keep it in stereo but i find many people use stereo tracks where mono would be perfectly suitable, and probably would sound better in the final product that way so yeah
^^ADD sidetracking... somewhat related advice though i think
Hope it helps.
