+1 to pinkunicorn's response. To add on, given that there are 10s of thousands of challenges around the world, you will still get blue marks in areas where you have not cached and for which you have not run many challenge checkers. And that makes sense from a server efficiency perspective -- why invest server time checking whether you qualify for challenges located in areas where you may never go.
But, if like me you travel a fair bit, and always look at challenges located in your destinations, then the server will start chipping away at the challenges in more areas.
And, although the checkers are an extraordinarily valuable service, they are not 100% accurate sometimes, usually because of lack of clarity in the challenge description -- and mainly with older challenges that had less oversight in their creation. So I always do a reality check to be make sure that the checker is saying something that seems right. The errors are very very few and far between, but they can happen.