From Piper Chieftan manual regarding engines

Come across another thing that has puzzled me (big surprise).

Here's a transcript from the manual:

"Under full throttle operations (such as takeoff and climb) the engines of
the aircraft have been adjusted to provide 43 inches of manifold pressure at
seal level and standard temperature. It is possible to read higher (up to 49
inches of manifold pressure) or lower than 43 inches of hg. manifold
pressure when ambient temperatures are higher or lower, respectively, than
standard."

I tried to explain to myself why the MAP would go up with higher than
standard temp, and down with the opposite situation. I came up a blank.

So, I'm asking the group. Why does this happen?

TIA.