I don't want to come across all "both sides do it" (oh God!)...
... but having been involved in one of these big tendering wheezes, I've seen a mindset that can only be described as conspiracy thinking between the client's managers and the consultants. It goes something like this: "You guys have done loads of these jobs, right? You're men of the world-- if we tell the sodding beancounters how much this thing is really going to cost, they're going to walk away right now. So you scratch our backs and we'll scratch yours: you go low on your estimate and take an, ahem, flexible stance on any change requests we lob your way, and we'll be likewise flexible on signing off on any overtime requests you need to get the thing over the line. How does that sound?" Except not said in so many words, of course. Certainly not in any way that could be taken down and used against any of the parties involved, nudge-nudge, wink-wink.
Of course, doing business this way basically means the company's IT department is in a conspiracy with an outside consultancy against its own management and finance department, and all those estimates, requirements documents, lists of "deliverables", timelines, and other precious artefacts of the tendering process are so much junk from the get-go. The project is off the rails from the start because the project scope is a fiction concocted to get sign-off and nothing more. And so it ploughs on, burning money, until the political embarrassment becomes too much and the managers call a halt going, "it was the filthy consultants! They made us do it!" It's a win-win for the IT managers, see: if the gamble pays off, you collect the ass-pats due to one who successfully delivered a critical infrastructure project; if it doesn't, well, it was only someone else's money anyway and you have a handy scapegoat to blame. "It was the consultants!" Yeah, that wouldn't ring so hollow if the exact same consultants weren't back six months later working on the next white elephant...