That's very true, and there are a lot of examples of artists that are widely recognized as OHW but technically are not. A couple of classic examples are Human League and A Flock of Seagulls. Most people think of these bands as one hit wonders and many lists of OHW have them on it, even though both bands had other hits. AFOS had the smash hit, "(I Ran) So Far Away" but they also charted with "Space Age Love Song." And with Human League, everyone and their granny remembers "Don't You Want Me." The band is virtually synonymous with that song and it makes a lot of one hit wonder lists. But Human League actually had another #1 that is often forgotten:
https://youtu.be/s1ysoohV_zA?list=RDs1ysoohV_zA
Rev, I may be wrong but I think what happens is that a band , or artist, becomes so intertwined with one song that it almost defines their career, at least in the minds of many. Kinda like an actor or actress who becomes so well known for a certain role. Carroll O'Connor will always be Archie Bunker, despite the fact that he had other great work, including In The Heat Of The Night. One would be hard pressed to find a list of OHW that didn't include Cutting Crew, AFOS, or Human League, because that one big hit overshadows everything else the artist did in the minds of a lot of fans. But you are absolutely right. They really aren't OHW.