Mainly because of the borders. So if you understand the borders getting cut off thing, you've answered your own question.
Although even on a non-border enhanced photo, you could still end up with part of the photo cut off if you have them printed in a different size than what you saved them as when you were doing the CD. Chances are that won't affect your overall photo to the point that it is noticeable, but it might if you have things near the edges of the image.
When you pick different sizes on the Photopass site, it shows you what happens to the different size photos - the ratios between the sizes is not the same and it shows you a little pop-up of the difference between what your photo will look like as a 4x6, 5x7 and 8x10.