Jet stream has been particularly strong of late. I fly the SEA>SAN>SEA route a lot and it's been noticeable - although the headwinds usually come on the Northward journey, not South. I'm no expert, but it sounds as though they didn't take that into account when determining how much petrol to put on the plane in Seattle. Better to figure it out late than run out of juice in the ocean. Or possibly it was just a ruse to keep people calm to the fact that, after takeoff, something serious was detected.

Not excusing the poor experience (I feel for ya, have been there), but keep in mind the alternatives, heh.