We say 6 weeks because the vast majority will develop antibodies and/or symptoms by that point, and because it's so uncommon. Tying all that together, it would be really unusual for someone to have syphilis and not either have symptoms or antibodies by then.
If someone were a known contact to someone with syphilis, I'd recommend testing out to the 90 days, or if they were pregnant or HIV+ and had a high risk encounter, but for most people, 6 weeks is fine.
Other websites, generally speaking, are giving info for a wide variety of people, and can't get really specific for each person reading. We can. :)
Good question. :)