When Apple introduced Passbook in iOS 6 they billed it as a way to store all your tickets, coupons, boarding passes, loyalty programs, and other cards all in one place. It's hard to imagine they expected that place to one day be Microsoft Wallet, but thanks to Windows Phone 8.1 that appears to be exactly what's happening.
Apple code-signs their Passbook cards, and it's not yet clear if Microsoft is simply accepting Apple certificates or accepting any pass regardless of whether it's signed or not.
Apple also provides a push-notification-based service to update cards, like gates numbers on boarding passes or balance info on Starbucks cards. Windows Phone probably can't hook into that system so, if updates are possible, there'd have to be some Microsoft-specific support from the supplier, or laborious polling process in place.