Just because a radio station advertises themselves as 70's and 80's doesn't mean that they're restricted to those two decades.
Hell, 1st Wave advertises themselves as 70's and 80's and I've heard a few 90's tracks on there ("Groovy Train" by The Farm and U2's "One" to name a few). They even play David Bowie's "Space Oddity", which came out in '69 but wasn't a hit til '72.
Yesterday, 90's on 9 even played "Maria Maria" by Santana, and it came out in 2000.
So why do stations do that? Simple: to keep things interesting, and those songs amazingly 'test well'.