This is because those sizes have different aspect ratios. The 3x5 matches the image, 5x7 does not.
If you put the crop scissors on for the 5x7 you would lose part of the image which I'm sure you would not want.
The larger print is in fact 4.2 x 7.
Terry
Terry is absolutely correct, but the decision to make a full 5 x 7 and a full 3 x 5 does depend on whether you can tolerate the content.
The girl's face, no good, too cropped. The scenery shot, up to you.
Fred