OK, antecdotal: Bob, in the Nursing home and can't remember his kids or their names, but when we start playing sings all the words to songs we do as instrumentals,and then asks for a specific song, and sings the chorus for us. Ed, age 100. Has trouble with the great grand kids names, but asks for tunes he heard in the Army during the Great War I, and remembers all the words. Not sure this is anything other than long vs short term memory, but I think your premise is true.