well this is a bunch of BS. Track builders today are installing 15A breakers, to match the 14 gauge wire they are running in the walls, that then attach to a 15A outlet. Builders are not installing 20A breakers or 12 gauge wire in houses to save money, unless you pay extra for that.
Now if you see a 15A outlet wired to a 20A breaker, most likely that is because the original 20A outlet went bad and the moron homeowner installed at 15A outlet to save $1.50 at Home Depot.
15A outlet --> 14 gauge wire --> 20A outlet = Bad
15A outlet --> 12 gauge wire --> 20A outlet = Bad
20A outlet --> 14 gauge wire --> 15A outlet = OK
20A outlet --> 12 gauge wire --> 15A outlet = OK
20A outlet --> 12 gauge wire --> 20A outlet = GOOD
15A outlet --> 14 gauge wire --> 15A outlet = GOOD