Last year Boeing built only 10 747s yet they have a backlog of about 30 craft. This year they have three done. Doesn't it make sense to build them quickly and get them out the door? Why do manufacturers drag each plane out. If I'm an airline, I wouldn't want to wait three years for an airplane that is already getting obsolete.
And if the market for used planes is so tight, why doesn't Boeing just build a couple extra 767-300s sans engines. They probably won't lose money on them and if they are sitting around waiting to be used someone would probably buy them.