I do not know the source of the data or how it is stored. Please keep that in mind as I post this supposition.
It appears that the stats at any one level include those of all prior levels for the individuals at that level. If the posts that were analyzed are not for a narrow time period, the output is thus for the invidual who reached a certain level and not for the level itself. For example, the chart shows 275 friars. If the posts analyzed cover a wide date range, perhaps some of those at higher levels also posted while they were at the friar level. Those XP/posts are attributed to the higher levels, though they were actually made while the individual was at the friar level. If the data were available, it would be interesting to see if a certain level is more active than another. As an individual progresses up through the levels, does the XP/post change for a certain level?