Is your retention measure vs OSM activity in general or HOT specific tasks you determine? I suspect that you measure the total OSM activity. Otherwise, I would also consider this as a biased measure of the OSM retention.
Your MM sample is too small and the time span too short to conclude at this point about the trajectory of the contributors. Yes it would be interesting to measure over longer periods. Measure of retentions up to 6 - 9 months would help to analyze further the trajectories of contributors.
It would be interesting to compare the trajectory of contributors that participate to periodic meetings with others, to see if they have the same mapping profile, how their volume of objects edited progress.