If I were to guess, based on what you're saying, it's probably due to the photo's being cropped to fit the size you are printing. Chances are your camera shoots at 4:3's ratio (4 units wide x 3 units tall) images, so if you are printing 4x6's (3:2 ratio) some will have to be cropped off the top and/or bottom to fit the image. Since you are using PS, crop your images in PS to the size you want to print to be sure you get what you want. That goes for any size... Some consumer online printer (shutterfly.com and, uh, forget the other one we use at the moment, oh snapfish.com) have "digital" print size, that comes prints some weird size like 5.333" x 4" prints (4:3 ratio)...

BM