Can barely used rims be concaved?

The other day I bought a 1992 Schwinn High Plains. The bike is in A+ condition. The man I bought it from was 70, the original owner, who said “I was in my late 30’s when I bought it”, which Is accurate “and it’s been sitting in my garage forever”. I know looks can be deceiving but I have confirmed by catalogs that this bike had its original 32 year old tires. And the chain is what Schwinn put on from the factory. It is not stretched either. So this bike has not even had enough miles on it to stretch the original chain or even wear out the stock tires. But what is interesting is, the rear Araya 26 is somewhat concaved. The front is perfectly flat. I find this really hard to believe that this bike has been ridden nearly enough to do this. How is this possible on a bike that has never been ridden enough to wear through a set of tires?