The problem is that if they write the spec for the new system before they do everything else then they'll have to stump up the full cost of the system and, more importantly, the full cost of any changes they want to make cos they wrote crap requirements in the first place. My experience of government projects is that they get developers/suppliers contracted on projects with a low spec. (and hence low price). This has two advantages - they look like they've got a good price and any cost growth will likely fall on a different administration. Then, they try to grow the requirements without paying the full cost by using a range of techniques, the main ones being threatening suppliers with not getting future contracts, using delaying tactics to drag the contract out at the suppliers' expense (££ and reputation) or going legal very early and pointing out that going to court would be more expensive than just doing the additional work.