The internal temp of the grill is the internal temp of the grill, no matter what the outside temps are. Set your grill at the temp you want it at, if you want to cook at 225F set it at that, it doesn't matter if it's 100 degrees outside or 15 degrees. It will take more pellets to keep temps the colder it is.
My wife does the same "math" with our thermostat. We normally keep our house at 68 in the winter. Some nights she will say "its supposed to get really cold out tonight, we need to turn the heat up a little"... 68 is 68 no matter how cold it gets outside.