My experience is that the less expensive options service providers are generally setup for the "average" snap-shooter, who doesn't complain if their image is slightly cropped or whatnot. Or, they're staffed by people who don't really know what they're doing or don't know anything about photography or digital imaging... or both.
The most useful explanation I received from one provider amounted to that they're not in tight control of where the paper path runs in the printer, and therefore, the image can be mis-aligned by a certain amount within the machine - so they automatically apply a 2% magnification to account for it (otherwise, in theory, the image could miss part of the paper, depending on the tolerances). I took this to mean that they basically don't keep real close track of the calibration of the machine for the cheapo prints... (which I assume is a lot of labor, and would drive up the cost of the prints - which doesn't work for a budget outfit - or, say at the local pharmacy or whatever, they don't even know how...). I gotta figure those big expensive machines can be run in a way that produces perfect prints every time, but... what do I know
The higher cost provider I've worked with does what I'd consider a perfect job of it, if I've done my end right (sized properly, etc) - but they don't print small prints, and are quite a bit pricier for things that aren't critical.
Frankly, I've had much better luck at the local grocery store than I had at the local camera store. In fact, I've been disappointed with all of the professional outfits here in Austin - the quality of the work that I've received just doesn't match the price I paid, at all... I just do things mail order, now, and pick the provider based on what I need out of it...