Joseph – thanks for the compliment!
It has to do with different mentalities and societal norms. I have the feeling that to an Israeli Yerushalmi Chassid I may look modern even though I am very tznius imho. My hair is longer than your typical Israeli Chassidish girl’s is, and there could be undefinable style differences – things that I may not even be able to say what they are.
Basically, the fact that I don’t look like an Israeli Chassidish girl (since I’m not one) may make me look modern to them. It’s the same way that if I see a girl or guy dressed in a way that is typically considered “modern” then even if they are technically tznius, I may automatically think of them as more modern than someone who is dressed more “yeshivish” and while I may assume they are makpid on halacha, I might not assume that they are makpid on things that are not strictly halacha but that Yeshivish people are makpid on.