From a programmer's perspective I would assume a basic algorithm exists where your search is partially matched, then shown in reverse order (most recent first).
If they're clever they might match other factors from your profile against the job requirements - e.g. if you have a required qualification, or previous experience.
As it's been a while since I applied for a job and the last job site we built was over a year ago, I can't speak for 'most job boards'. If I were you I'd give a few a spin and see if you can work out what's going on when you search. Try some non-logged in searches, then set up a profile and try again, see if the results are different.
Martin
