Totally Covered

The Gemorah in Yevamot (78b) says the law of Rebbe Yitzchak that only a a substance found on the majority of the body that the person is particular about constitutes a חציצה apply to the majority of the body but if it found on the entire body even if the person doesn't care, it will still be a חציצה.  The question is where does this distinction come from?  Furthermore, generally רובו ככולו so if a substance on the entire body will be a חציצה so too on the majority of the body?  The Briskor Rav in Mikvaot and the Sfas Emes in Sukkah both say the halacha of a substance on the entire body is not because it is a חציצה but rather because it is not considered as if the person went into the mikvah.  Since the person is covered from head to toe, they did not enter the mikvah.  That is why the immersion is no good, not because it is a חציצה.  Then one can't say רובו ככולו for there is no din that כולו is a chazitzah just that the person in that scenario did not enter the mivkah. The Achronim ask why is there on rabbinic gezerah  on a substance on a minority of the person that they aren't particular about so as one doesn't get confused when one's entirety is covered?  According to this approach we can say that this is not a halcha of chaziztah, it is a lacking in entering the mivkah and the rabbanan only made a gezerah when it is the same din.;