A friend pointed out an interesting English quirk to me the other day: Why is a prisoner in prison but a jailer runs the jail?
The answer lies in the suffix, -er, which derives from a Proto-Germanic suffix -ărjo-z, which was added to nouns and meant "a person who has do to with [noun]." Originally the main purpose of this construction was to denote a persons job - a jailer work for a jail. In Modern English the meaning expanded to also denote something a person does that is not necessarily their profession - a runner runs, but it's probably not their day job. The definition further evolved into something like "a native or inhabitant of," which is where we get New Yorker or southerner - and prisoner.
As a side note, there is an obsolete definition of prisoner that meant "person who runs the prison."