First, there has not been a definition change or re-evaluation of editor counts. Current counts are wrong. Let me explain why all months display lower counts: on every monthly run of wikistats nearly all data are regenerated from the dumps. This is on purpose. This way new functionality and (rare) bug fixes apply to all months since the creation of the wiki. The other side is that when stats scripts or dumps are broken reports will show wrong data for all months, which is what happened now (a hybrid situation with data retention and update runs would complicate processing further and easily be a source of errors itself). I put up a notice on the stats pages.
I am investigating. So far test runs have been inconclusive. My first priority is to find out whether this is caused by a change in the dumps, or the scripts, or a config change on the server. The latter is most likely. In the past month Wikistats data and script were moved to a new server, with a number of modifications to overall configuration and shell scripts.
I will keep you posted on any new findings. My apologies for the inconvenience and confusion caused by this mishap.
Update June 17
The problem has been analyzed and fixed. A few weeks ago, during substantial overhaul on stats scripts (to add new metrics) an error had creeped in, which was not recognized during tests: as a result far too many articles were flagged as redirects, resulting in far too low counts for articles and editors: redirect pages are not counted as articles, and for consistency, not taken into accounts for edits and editors (*).
Now it still will take about 7-10 days to reprocess all dumps. Slightly delayed further by scheduled maintenance on the stats server, which is still ongoing.
Thanks so much for your patience.
*: This is of itself a point that could be debated, but this is how it works or is supposed to work. For one it prevents skewing of edits per article metric.
Update June 26
All 800+ wikis have been updated now.