That could be because of cheaper production costs or because Apple has seen iPad sales dropping and traded some of its margins to sell them at lower prices. As a for profit company with very good margins, it only makes sense that Apple would continue to maximize that and not let it slip a lot, even if the gains may seem minimal to an outsider. It’s also the same reason why Apple continues to sell Macs at the same price a at launch time even years later without any hardware updates, even though Macs are a small percentage of its total revenues.