it pretty much has to do with the density of air and amount of water molecules in it.
if the air is thin (high altitude) and/or the there is a lot of humidity (more % water in the air) you need a smaller jet (which controls the fuel flow) to keep the proportion of gas/oil to air. in denser air with less humidity, there is more % oxygen so you can use a larger jet (more gas) to keep the fuel/air mixture balanced for the ignition (explosion in front of the piston).