Yeah, that's what I figured. It's not like it makes that much of a difference, anyway, since I doubt anyone will be too concerned about prefects are anything like that with the looming threat of the Wizarding War over the school.
James Potter was one of the exceptions to the Head Boy being a former Prefect. But, yeah, normally they seem to be students that were formerly Prefects.
It seems to me that the Potters are very privileged.
First James becomes Head Boy without being a Prefect, then he marries Lily after being a total prick to many people (even though Severus was cut out of her life entirely for one f'ing slip-up brought on by his Slytherin buddies), then Harry becomes Head of Auror Office without even taking a single N.E.W.T.