busses volume output question.

I don’t understand why group busses give me a different volume output than the master buss.

Consider this scenario:
you have a full rock mix in your project.
You have all the drums grouped in a group buss - you have clipped all its single channels so that the
drums buss group now gives you a volume value of +10.

BUT if I put a meter on the first insert of the STEREO BUSS, and press play, with the whole song and instruments as well,
now it says the peak value is i.e. +4, not +10 or above (like on the drum buss group). It is always lower.

I can’t understand this thing.

Thank you in advance.

Does changing the pan-law make a difference ?

Yes, same behavior.