It seems Windows Phone store is being prepared for arrival of Windows Phone 8.1, which is quite natural though given, it may be showcased to the world during BUILD 2014.
A Reddit user has posted screenshots of some of the apps which were caught with mention of Windows Phone 8.1 compatibility, but this disappeared soon. So, may be this appearance was due to some kind of testing going on. Windows Phone 8 apps are supposed to be compatible with Windows Phone 8.1 devices according to leaks, while Windows Phone 8.1 apps will be compatible with Windows Phone 8 devices only when they run Windows Phone 8.1 update.
Source: Reddit
Via: WMPU
