People often talk about requirements being defined, but then you find that once they are, the next step is satisfying those requirements. However, what is missed is the step in between - defining the specifications.
The analogy is they wanted a wall, so a wall is built, thereby satisfying the requirement. The customer then says you've built a three foot wall when what they wanted was a six foot wall. Had it been specified in the first place, they would have got exactly what they wanted.
Of course they should also have specified the type of bricks and where they wanted it etc...
Then the other big problem is design change half way through, but that's another story.