Different App stores? depending on o/s, region, carrier?

[the whole taco]

for example, I can download a particular banking app for my Surface Pro 3 running 8.1pro but it isn't it the app store for Windows Phone, and vice versa.
so how much, and why, do different o/s affect availability? any truth to the rumour that the carrier affects what's available.
and region? I can understand some differences but generic apps not region specific should be easily available?
are there other 'app stores' I should be browsing outside of the pre-installed ones?
tia.
(am in Oz)

 

[PepperdotNet]

There are currently two stores. One for Windows and one for Phone. Universal Apps appear in both if/when the developer publishes to both. An app designed specifically for one or the other only appears in the appropriate store it's published to.

The Store is filtered depending on your regional settings. App vendors can specify which region(s) their apps are available in.

The carrier has its own filter that affects its "recommended" or "featured" apps but otherwise doesn't affect the rest of the Store.