With the Scarred Lands it was like this:
3e was new, the PHB and DMG had just come out, and suddenly the first real d20 release is the Creature Collection - a bit BEFORE the Monster Manual. Now, obviously SSS must have had access to a draft of the MM, otherwise they'd have even more errors in their monsters than they do. I bet they anticipated the disappointment the general public would have with the MM (fewer monsters, poor formatting, too few illustrations, illustrations that stunk) and figured the best thing to put out is a monster book. Fast-track it to get it out before the big MM and you've generated some buzz. People are going to snatch it up to get their hands on some monsters.
You KNOW you have an instant audience for your product, so you advertise what you really want to push - the setting. Granted, it's just bits and pieces here and there, but nevertheless, some of the Scarred Lands mythos seeps into the book. Lather, rinse, and repeat for the next most obviously sought after book (well, this is before the prestige class boom) and you've got Relics & Rituals (with a HUGE assist from open call writers).
Now then, two books in, one of two things have happened. People have read the bits about the Scarred Lands and become intrigued enough to purchase setting books, or people have determined they have no interest in the setting. It's a good plan. Come up with two must-have books, try to do it as cheaply and quickly as possible, and build some buzz.
Fantasy Flight games takes a slightly more traditional path with their settings - do a book for players that is half crunch and half fluff, then do a DM's book. But they're going for niches. You had a sci-fi futuristic campaign that was unlike everything out there in Dragonstar, which did build some buzz for them and probably sold a book or two more than they would have otherwise. Midnight went the route of whoring out Tolkien for all it was worth in perhaps the ultimate "what if" of the books. That gets some interest, and enough new things exist in the crunch side that you'll either buy into their book for that and get hooked on the setting, or buy into the setting and get hooked on the crunch.
Of course, consider the sources. WotC has a built in following based on the D&D brand name. They do enough generic crunch books (Manual of the Planes, Savage Species, Book of Vile Darkness, etc) that they can swing supporting Forgotten Realms (although it seems that hasn't been the mad bank they expected it to be).
S&SS may have the long arm of White Wolf and a host of other d20 publishers under their umbrella, but at the heart, S&SS only does Scarred Lands. When your revenue comes from the setting books only, you needs to spread it out into a lot of little books that are setting light and crunch heavy to pull other people in.
FFG pushes most of their support down the line of the crunch books, things like the "Path" books, Traps & Treatchery, etc - books with wider audience that generate higher revenue. On occasion we see a new release for one of their two campaign settings, but most of FFG's time is spent in pumping out new broad-appeal books.
The reality of it is a company that does not rely on sales of their setting material to turn a profit can be a bit bolder when it comes to the type of fluffy campaign material they release. They can afford the one or two book method where the crunch is about 1/3 of the material in the book. Companies that are tied directly to their setting must come up with a lot of new rules and crunch that an be dropped anywhere to entice people into buying the setting books.
Compare the FRCS to the Creature Collection to the Starfarer's Handbook/Midnight and you'll see the pattern develop.
Even that I changed heavily to suit my own nefarious needs.
