Why it is said that Finland has a particularly good education system, but Finland's performance on international mathematics competitions is quite often at relatively intermediate level?
"Why it is said that Finland has a particularly good education system, but Finland's performance on international mathematics competitions is quite often at relatively intermediate level?"
Because the first part is a comparison of population averages and the latter part is a comparison of population extremes. If the distributions vary in shape, than you can't expect the inferences to be identical.
In addition, given there's a difference in population size, one can expect (all else equal), larger countries to perform better in Olympiads.
P.s. Your monicker is making me feel like quoting Highlander.
My general idea of math results is that it only shows how good one is at that thing, and it's not very easily comparable.
For example, you could be good at calculation and go extremely well in tests (that could be checked with a calculator) but be very weak on concepts. Likewise, you could be very good at concepts but not be good at, say, answering multiple choice questions. And, again, you could be good at multiple choice questions but then not know unique formulas and ingenious theorems that could help on a math contest.
People are good at different things and math, and especially evaluations in math, are different too.