Most of the capsaicin is in the pith. On some peppers the pith is not really differentiated from the flesh other than the ribs that jut out and where the seeds hang. On the reaper and some of these other hot peppers, the pith is a very noticeable membrane covering the entire inside surface of the pepper. The flesh of the pepper is actually very thin, so you end up with with a lot of pith and not that much more flesh. That's how the percentage can get so high.