Of course, this means that the vendors have an incentive to pad the bids to account for "unknown unknowns" and use weasel words in the statement of work so that deliverables have a clear-cut boundary. And it means that the purchaser has an incentive to play dumb so they can get the most for their money.
While economists would likely discuss this role-playing using Game Theory, I'm going to take it a step further by examining this behavior in the light of Game Show Theory:
Fun and games
Obviously, I'm somewhat jaded about (OK, really jaded about) adversarial negotiations and inflexible purchasing processes. The games may save the client some time, but the cost is trust. And without trust, Agile projects don't really work. So you're stuck with Waterfall, which is unlikely to deliver something effective, efficient or truly fit-for-purpose.