I grew up in Buffalo, and moved here directly from Illinois, and commuted exclusively by bike.
The first year I was here, I was simply amazed that it was never cold. Ever. Shorts were all I ever wanted to wear. It was the mildest weather I'd ever experienced.
Then I acclimated after about a year, and now I'm a wimp with multiple layers like the rest of you.
There are more Midwestern immigrants that move to Seattle than California immigrants, BTW, contrary to the common misconception. You are probably seeing folks from Illinois.