My freeze is not frost-free. Why is frost in sealed packages? What causes it?

Why and how does frost get onto food in a sealed package in a freezer that is not 'frost-free'?

Water evaporates from the food and condenses on the colder wrapping.

Thanks DrTom for your answer. I assumed that the moisture came from inside the package, but what is the difference in the surrounding air that is different from a frost free freezer?
If the temp is the same in both freezers, why does one condence the moisture in the package and one does not? Something's up.

I have always been curious why.
Jack

There's always moisture in the air in the freezer as well as in the food inside the package.
The frost free one is continually removing moisture and sending it down a little drip tube at the back to a pan where it evaporates (that's the usual method anyway). So in a sense frost is there, just not there long enough to show.

If a package is colder when put in a warmer freezer frost will form on the outside of the package, plus when the freezer goes into defrost it vaporizes some moisture and that condenses onto the outside of packages.