Sprockets and rails ???

Quite often when looking at the for sale ads I see "Rails 90% sprockets 20%"
With a mismatch in wear like that wouldn't that be hard on the newer parts?
What happens when you now replace the sprockets and have 85% rails?
Is this normal for these parts to wear at different rates, or is it just owners replacing parts at different intervals???
Just trying to continue my education....Thanks