From GC's site: total caches = 1844, sum of countries = 1843.
Either you have found a cache not assigned to a country or, more likely, there is a corruption in Geocaching's database, which does happen occasionally. I suggest you contact them to look into this. You could also download your finds to work out which cache is the problem, but it is probably easier for GC to look into it. Once they have fixed the problem, Project GC should update a few days later.
