You are giving them (mostly) the same information, but the presumption when not registering is that they will only store the information as long as necessary for their internal purchase history purposes, whereas an account remains "exposed" to the web, for a seemingly indefinite period of time.
That is likely the presumption for many people. It is in general naïve I'd have to say as your information is kept for a certain amount of time for returns or disputes and longer for archival and reporting purposes.
I don't have data, but I'd veture to guess that most non-account data is stored nearly as long - if not as long - as account data. (I'm thinking primarily of ecommerce applications here. Services like pastebin are quite different.)
