I manage a one hour lab, and both above replies are spot on. I'll try to explain it another way too...
Look at a piece of 35mm film (negitive) The
aspect ratio is 2/3. That means that one side is 50% larger than the other. So, a full frame print from 35mm would be 4x6, 8x12, etc. Ever notice how a little bit gets cut off an 8x10 also?? Thats why. The industry standard is 2/3 and probably always will be.
Now, a digital pic is NOT 2/3, and actually varies on the equipment used. For sake of arguement, lets say it's 3x3. When you enlarge a 3x3 equally on all sides it becomes a 6x6, 9x9, etc.(or anything inbetween.) So, if we enlarge a 3x3 to 6x6, then make a print on 4x6 paper, you're going to lose some, one inch off each of two opposite sides.
The printer will automatically alighn itself off the smaller side, thus cutting off the excss pic from the top and bottom (assuming you shot a landscape (horizontal) picture. The tech can adjust for this and manually decide what will be cut off and what won't. But, most labs won't do this because it means taking more time and correcting each photo. Find a lab that will.
I get lots of trail cam pics and fix everyone of them. If you get a nice buck pic, I've even been known to slip a free 5x7 into your package
Hopes this helps.