I'm someone who deals with Public Sector tenders as part of my day job (tech in my case). Firstly, the scoping of contracts within the public Sector is usually poor, their procurement is disconnected from the business and invariably, there are too many contractors and consultants out to cover their backside/feather their employers coffers. Too many talkers, not enough decision makers
Secondly, they are moving to fixed price contracts but don't recognise that fixed price adds a risk premium to companies bidding. If the bidder is not desperate, he is going to add 20%+ to the tender to cover themselves
Thirdly, they do not really get change management and the fact that it might be fixed price, but it is fixed from a specific set of requirements and if you change those requirements, you are going to get charged. Change in an existing contract usually costs more then if it had been factored in from the start
Lastly, from a budgeting perspective, my experience is that the concept of a risk reserve to deal with change is non-existant and not understood