Numbers are recalculated every month. I know, probably not what you would expect. My guess is that there are some users that are now identified as "bots", Ping @JAllemandou that can add more detail

I know that the numbers are recalculated, and small difference (few editors) would not be a surprise. But the old and new curves are around parallel with 5-10% difference. I believe that there is a methodological change behind this.

(Sorry, the files are apparently expired since yesterday evening, and not available. I will upload them again later today.)

Hi @Samat, thanks for reaching out.
It would be interesting if you could upload the files again, and also possibly confirm the URL you downloaded data from, as my tests/checks don't show differences that big.
I have checked the number of users only editors for huwiki over 4 years, looking for differences in our last 3 snapshots (we call monthly recomputations snapshots), and while there a very small deletion-drift (difference due to pages being deleted, as they are excluded from statistics computation), they are really not a 5%/10% change, more like -0.05% to -0.10%, and only for 3/4 month before last month.

Thank you @Nuria and @JAllemandou. I tried to answer your questions below.

The source file (csv) downloaded on 7th May:

The source file (csv) downloaded on 23rd June:

The two series in one ods file:

The minimum difference is 0%, the maximum difference is 20%, the mean difference is 4.6%, and the median of the differences is 4.5%.

(I didn't know, it is so easy to add files here.)

Thanks a lot @Samat for the details.
Indeed you were right the difference is to be accounted for a methodological change. I'm sorry not to have noticed right away.
From the month 2019-05 onward, we have changed the way editors were computed by removing the edits on deleted pages.
We did this to be more homogeneous, as other metrics (edits and edited-pages for instance) were already computed with deleted-edits removal.

I've been looking for an announcement of that change as I thought we did one, but couldn't find any, so maybe I imagined we did :(

I hope this change doesn't prevent you from running your analysis.

@JAllemandou let's please document this here: https://wikitech.wikimedia.org/wiki/Analytics/Systems/Wikistats2/Metrics/FAQ and make sure that the metric page on wikistats is accurate (i think it is) : https://meta.wikimedia.org/wiki/Research:Wikistats_metrics/Editors, it will be good to record the date of the snapshot where the definition changed.

Ping about docs

Done ! Sorry for the delay.