From what I can tell (I'm the owner of the house), the first 6 feet of the floor were built slanted. A friend of mine with some contracting expertise has suggested that the room was likely a porch that was later closed in; such a slant would be typical for an exterior floor to shed rain. The flooring also changes slightly in how it's nailed down around the 6 foot mark, suggesting that the underlayment might change there too.