Tznius is primarily a mode of behaviour. Mode of dress is an important part thereof, but is not the entirety of it. Halochas must be observed and I am not suggesting otherwise. Nevertheless, there are many aspects of tznius that simply cannot be legislated and therein lies the difficulty.
One could keep all the halochas, but still be un-tznius. Example, a woman who keeps every chumra known (and unknown) insofar as she dresses, yet chews gum like a cow chewing its cud while drooling. Plainly this is not what we expect from a Bas Yisroel. Or, who blocks up the street while fetching her kids from cheder and ends up inconveniencing other people. Or, who screams like a fish-wife.
With men, it could be a case of inconveniencing other passengers on an airplane because they needed to say a 20 minute shemoneh esrah. Or, driving an over-the-top car in order to show off one’s wealth. Or, for that matter ostentatiously keeping chumros to show the world how frum one is – if you’re sincere do it quietly!
My experience has been that dressing tznius is often a direct outgrowth of behaving tznius, more than as an obsession with the technical requirements of dressing tznius. Notwithstanding, that the halachas must be observed regardless.