Ronney,

This is what the Kollel answered for a similar question-

The Teshuvas Radbaz 1:572 explains why we say Nevorech in benching and Borchu...Hamevoroch for reading the Torah.

(a) One cannot read from the Torah unless there is a minyan. Since there is a minyan one must mention Hash-m. It is only the person who reads from the Torah who is obliged to read whilst the rest of the community are not obliged to make a berocha. The berocha that he makes is based on the verse (Devorim 32:3) "For when I call the Name of Hash-m; ascribe greatness to our G-d". This means that he says to the rest of the congregation "when I bless Hash-m you should ascribe greatness to our G-d". This is why he says to them "Borchu", because he is reading from the Torah but he also wants them to bless Hash-m.

(b) So for Kerias Hatorah it is not appopriate to say Nevorech because it is only one person who is actually obliged in the berocho. He has to say Borechu. However when he says Borechu it looks like he has excluded himself from the klal. This is why he also says Hamevorach, because this way he re-enters the klal since he says that Hash-m is blessed by everyone.

(c) Birkas Hamazon is different because they have all eaten together and they are all obliged for Zimun. One person makes a berocho for all of them; who are all obliged; so he says "let us bless Hash-m". Because he has not excluded himself from the klal, he does not have to re-enter by saying "Hamevoroch".

Dovid Bloom