CheckBook Pro 2.5.6 may not actually be released on the App Store at this point. With the approval process being so lengthy, we nearly have version 2.5.7 ready, so it's likely that we'll end up timing the release for 2.5.7 up so that it hits at the same time on the App Store as on our website.
Also, we've tried to always make it clear that updates normally get released sooner on our website—that's a benefit of buying directly from a developer (along with the potential for upgrade discounts later on). Those who purchased from the App Store can still install the newer versions from our site anytime and App Store licensing will be honored automatically in those downloads, it's just that App Store versions will never be capable of presenting notifications for when new updates are available on our site and the basic iCloud support isn't officially supported in non-App Store applications (full iCloud Drive support is available, though). Anyway, if you want to install CheckBook Pro 2.5.6 now, here's the link for where to go to do so:
http://www.splasm.com/checkbookpro/update.html