I will explain it without the ratio thing, cuz numbers seem to be messing you up!!
4x6's are a rectangular image. When you order an 8x10, notice that it is a slightly squarer format....that happens simply because they print only the center portion of your frame...so they are clipping a bit off each end. Same thing happens with 5x7 (though its less noticeable) 11x14, etc... for an 8x10, you actually must ask them to print a full frame image, which will be 8x12...then you can choose what portion (2") you can trim off, usually you have to sacrifice a bit. Sometimes you just cant sacrifice anything on either end, so you will be stuck with cut off images or just leave it as 8x12. You must stay aware of this cropping when you are shooting...if you look at alot of photograhers websites who shoot weddings for example, you'll notice some extra space on pics, that's often to acccomadate the clipping (cropping) that occurs with enlargements!
does that make sense to you?