Why do clients approve site designs just to complete change their minds upon delivery?!?

It is sad but drives home why we need sign off on the business requirements document. Significant changes v2 😀

After approved wire frames and approved prototypes, on the last sign off.... "That's not what I asked for!" Me: It's what you're getting!

Thats not a problem. Just charge them for extended changes.

Because they are not developers or designers. They often do not have the capacity (and experience in doing so) to fill in the gaps that a design, wireframe or technical document leaves open. Unless you've shown them basically a fully functional model of the product they just don't know what you're building. Regardless of the amount of discussion, signing off etc. Both your mental models are incomplete and differing from eachother.

And then there is experience gained while the project is ongoing... The truth that existed at the time of assigning it to you might no longer be there.