I'm sure this question has been answered ad infinitum on this forum (we really should have a FAQ), but I'm happy to answer:
There are two types of positions--
Categorical, meaning that you are accepted for the full training course in that specialty (e.g. 5-7 years for General Surgery) so long as your progress from year-to-year is deemed suffcient (meaning you'd have to be darned bad to get kicked out).
Preliminary, meaning that the program provides you a one year contract (usually for Internship) and you still have to find a PGY-2 position.
Most surgical subspecialties give you a General Surgery internship at the same institution as your Match program (e.g. ENT, Uro, Ortho), so you only have to enter the Match once for your Categorical subspecialty.
So the majority of prelim positions are filled by FMGs or people who didn't do so well in the Match and need to find a place to train for a year while they reapply. Since surgery departments always need Intern cannon-fodder, there are always plenty of prelim positions available. Prelim positions are often taken in hopes that a PGY-2 position will be vacated and being there counts as having a foot in the door--I'm sure doing a good job as a prelim Intern helps, but waiting for a categorical resident to drop out is a roll of the dice.
