These numbers and several others used to get a lot of queries here and in other Discourse instances back in 2014/15. There were so many new users wanting to get badges and we didn’t really trust Discourse. Some differences were indeed bugs so most of the calculations have been fairly well confirmed.
I checked some myself. I remember getting my data and tallying how many UTC days I visited Discourse. I had third-party stats from network monitoring software so I could double-check the Discourse stats. It would have been nice to find some big anomalies but Discourse was correct for my stats.
That doesn’t mean there isn’t a problem now. It just seems unlikely.
By the way, I still don’t have the badge because it is difficult to work to UTC when I’m as far from it as you can get on this planet.