It's simple really. You can use the same casing and fill the chemical solution with a slightly cheaper compound that will burn off the energy faster, hence 1400, you can fill it with more expensive solutions that break down energy slower, hence the 1500 battery. But, you can only peak out within the same casing after that you get the bigger batteries. Manufacturers dont want to spend THAT much money giving you the best product life, they want you to spend money on little upgrades and accessories. hence how they can offer a better battery with the same fit. same as buying a plastic ring, silver ring, gold, tungstung(the heaviest) ring... same size, different material, but weight beings to change even though its the same shape and form, and also taking up the same space.