Why is it spelled s-o-l-d-e-r when it's pronounced "sauder?"

I used to love watching my dad soldering stuff. I don't even remember exactly what it was he'd use it on - broken toys and electrical equipment, I expect - but it just seemed like a cool thing to be able to do. I don't own a soldering iron myself, but after reading this post I'm thinking maybe I should get one. The illustrations alone are worth a look. (via boingboing)