Vendors in their zeal to win the bid almost always, underestimate the requirement and under bid on the project. They jump into development without completely understanding the requirements and eventually end up delivering sub-par product. Then starts the cycle of what is in-scope and what is not. Soon re-estimations and negotiation for higher amount follows.
In my experience as a client you spend about 40% time explaining the requirements to developers and 60% time in constant negotiations with everyone explaining, that the required functionality of the "one line description of the feature" really is not as simple as they thought. I know the obvious answer is to give a detailed feature description during initial quote request. Well it is easier said than done. For one, you do not know or have time to detail the feature functionality, and secondly, it is not advisable to share so much detail of a project during quote request phase with multiple vendors. And even if you do, the offshore vendors just deal with so many quote requests a day, that they simply do not have time to go through detailed requirements.
I guess, it is just part of doing business with offshore vendors and one needs to be prepared to engage in long discussions related to scope of the project.