Why altitude changes the arithmetic
A helicopter generates lift by pushing air downwards. The higher and warmer it gets, the thinner that air becomes, and the less lift the same rotor produces at the same power. This is why performance charts, not seat counts, decide what a helicopter can carry on a given day.
In practice the difference is dramatic. A machine that lifts five passengers comfortably out of Pokhara at 820 metres may only be able to hover safely with two or three at a landing point above 5,000 metres. Nothing is wrong with the aircraft; that is simply physics.
Temperature matters too. A warm morning reduces performance further than a cold one at the same elevation, which is another reason mountain flights depart early.

