Android app updates go through the Market (or Google Play or whatever it’s called now.) I suppose the ones you looked at had sideloaded apps which had to update independently.
as for WP7, if you have the Marketplace tile pinned to the Start screen, it’ll show the number of available app updates (Windows 8 does this too.) If you don’t have it pinned, it’ll tell you how many updates there are once to open the Marketplace app.
I don’t know that this is the case, at least on Android, I don’t know about iOS.
Many apps get data from the developer’s website, but I have very few (Android) apps that are just a front end for a website. Of course some are because they have to be. IMDB, for example, is supposed to be a front end. But even then, the updates aren’t usually related to something having changed on the website. IMDB’s most recent update fixed bugs and added the ability to play HD videos on phones and tablets. Before that it was a new interface.
Many “front end” apps also have location services, barcode scanners and other features specific to the phone that need updates and bugfixes.
I’ve looked through apps that I’d expect to be front ends for a website, like IMDB, Home Depot, Office Depot, Pandora, Weather Channel, etc, but none of the recent updates are related to the website. It’s most often bugfixes for new devices, new translations, and new features that are related to the phone.
I’d call those apps “semi-front ends” because they have other features, but they actually seem to have fewer updates than the apps that do not rely on content from the internet. And the apps I have that are true 100% front ends, like craigslist, TVTropes, Ebay etc seem to get updates very rarely.