It is nothing to do with the Mercator Projection. It is because you haven't included China in your map. And the map you have in your quiz isn't even a Mercator Projection anyway. See here for more details.
Great quiz, given 5 stars (was a sad sight to still see them white ;) ) could ve done with a tad more time. 30 seconds, so ypu can have typos and think about a country or two
Just to challenge for the nitpicker's award, since Andorra is excluded, there should be a grey/white gap on the map where it is located - and I can't see it.
Though shouldn't Andorra be on the list too?