I have a theory - or a bare hypothesis really - that the things Japanese are amazed gaijin can do are the things that Japanese tend to worry about their own skills in or they feel are dying out. Chopstick skills are deteriorating in the young, according to one report I saw (too much western food/fast food), so the Japanese are extra-amazed at how well gaijin can use it. When that Canadian dude used some slightly uncommon keigo (滅相もない) and everyone went Oooh! that too might be a comment about the decline of correct language. Or even sitting in seiza - I have beaten Japanese at seiza gaman contests, but only younger ones (apparently PM Hara used to sit in seiza at all times - not many could do that now)…. Conversely, has anyone been told “gosh, you bow well!”?overthinker
In many cases non-natives understand Japanese better than Japanese understand themselves.