it is indeed not so "clear cut". here is a halachic discussion from Rabbi Mordechai Willig which sheds light on this

> Notwithstanding the immutability of the Torah's principle of modesty
> and its particular application to women, the precise details are
> subject to communal standards which often change and/or vary from
> place to place. This is true regarding some parts of a woman's body
> which must be covered (Shulchan Aruch Orach Chaim 75:1). Nonetheless,
> there are other parts which must be covered regardless of communal
> standards.
> 
> The Mishna Brura draws the line at the elbow and the knee (75:2). Some
> interpret "shok" (Berachos 24a) as the calf (since the thigh is called
> yerech), and include it in objective erva (see Chazon Ish Orach Chaim
> 16:8). Yet others imply that since the requirement to cover the arms
> and legs is das Yehudis (Kesubos 72a), i.e. a custom of Jewish women
> (Rashi), it may be subject to change (see Kaf Hachayim 75:2, Igros
> Moshe Even Hoezer 1:69). Sha'ar Hatziyun 75:5 disagrees.
> 
> However, a woman's torso is certainly ervah (see Rambam Krias Shma
> 3:16), and must be covered. Unfortunately, many otherwise observant
> women follow fashions, such as very low necklines, which expose the
> flesh inappropriately. Women who wear tight-fitting clothes which
> explicitly delineate a woman's figure are also in violation, as the
> Midrash, contrasting Rus and the other women, implies (see Kuntres
> Dinei Malbush Nashim page 12, 13).
> 
> The distinction between variable details and timeless principles is
> not limited to dress. It applies, in a more complex and nuanced way,
> to the definition of tznius in the Torah society. For example, public
> speaking by a woman in front of mixed audiences is commonplace in some
> circles and unheard of in others. For many parts of Torah society, it
> depends upon the place, the occasion, and other factors. Similarly,
> interaction between men and women, another subject of the Midrash
> about Rus, is also dependent upon local custom (Beis Shmuel 62:11, see
> Otzar Haposkim there). This includes separate seating, entrances,
> mechitzos, etc. Here, too, context is clearly critical.
> 
> It must be noted that the opposite of tznius is pritzus (Kesubos 3b),
> a term linked to one who breaks a fence (Koheles 10:8), and different
> communities legitimately build their fences in different places. As
> such, a garment, speech, or event can be labeled as pritzus in one
> place, but be acceptable in another.

from http://www.torahweb.org/torah/2010/parsha/rwil_bamidbar.html
  [1]: http://www.torahweb.org/torah/2010/parsha/rwil_bamidbar.html