If the cache is in the top 10 then it is very probably an antipode cache or something like that, and those are probably on an exception bookmark list for caches that by default are not included in statistics either on Project-GC or on geocaching.com. On Project-GC, you can go to Statistics -> Profile Statistics -> Settings and change the setting of the checkbox that controls this ("use official ignore list for traveling geocaches" or something very similar). By default, the setting is to exclude these caches.