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This is not a bug. You are using the "Published date" filter, meaning that a geocache hidden on 2000-12-31 (a year before) but published not until a couple days later is included, and counts towards an extra month.
You probably want to use the "Hidden date" filter, like this: https://project-gc.com/Statistics/TopHiddenMonthLoggers?filter_pr_profileName=&filter_prr_country=Slovakia&filter_prr_region=&filter_crc_country=&filter_hd_fromDate=2001-01-01&filter_hd_toDate=2001-12-31&submit=Filter
