Here is the answer for your questions.

Its all about eligibility for different study backgrounds for more details read this.
https://www.inspirenignite.com/what-is-g1-g2-g3-in-ccmt-and-how-would-i-know/
A common answer for why some IITs have less scores than NITs for M.Tech admissions is that it depends on IITs and course we are looking at. older IITs goes first and their core specialization is always higher. Students usually prefer core branches and leave out iits if they don’t get a choice of their preference. that’s why scores in IITs are sometimes lesser. I will try to make a complete analysis post on that.