It's been a few years, but I'm reasonable sure I actually deleted it and didn't just deactivate it. Or at least, that was the impression Facebook gave me. There was a big "you will lose all data!" warning as well as a cooling down period of a week or so, during which I could restore my account in case I changed my mind.

Perhaps you logged into Facebook unconsciously or through an app within that time frame? That is enough to stop the deletion process.

Perhaps, but unlikely; I didn't own a smartphone until January, and I've set up my browser to selectively remembering cookies for ages (I don't have Facebook in the whitelist, for reasons that should be obvious).