It has nothing to do with buying it on your phone. Microsoft has no method in place for that.
It's because you downloaded it when it was free. So basically, the original payment structure was getting it free, and then paying $5 as an in-app purchase to get the rest of the levels.
It appears with the switch to Xbox Live that they switched that payment structure to be $5 up front. So, if you already had downloaded it for free, you automatically get access to the full version because there's no differentiation between downloading a free app, and buying a paid app. It's kind of like when you download a free app that you 'buy it' for $0. So since it's already on your purchase history, you get free access to download it at a later date. Basically you just got lucky in this one specific case.