i believe that site only shows "reachable" nodes. which means only full nodes that are accepting incoming connections from other nodes. otherwise this site can not find them.
and besides many nodes aren't ran 24/7. many people are running it on their PC and you can't just keep it running all day.
Yes, but that situation applies to the whole world. So if in Japan there are 81 full nodes and maybe 80 more full nodes that connect and disconnect some time every day, the same would be probably true for France - so there in addition to the 450 full nodes shown on the page there would be ~450 more nodes than this page says.
It could be interesting to observe if Japan's node count had grown significantly in the last months - maybe before the "good news" it was almost unknown there and is now beginning to get traction. But I didn't find historical information on that page.