Yeah, I assumed that would be it too, but I checked and for reasons I'm not highly educated enough to understand, the Latin plural of "census" is "census" (unless its in the genitive case, in which case the plural is "censuum", which seems pretty rad but not strictly relevant to this usage) and thus in English the rarely used "proper" plural is too. Maybe it wasn't originally considered a countable noun? That'd be kind of ironic.