I do a lot of contract work in a different industry and not government contracts, and I was curious how it compares to that. 

The way it works with me is that my submitted bid is what I will get paid, period. 

I have to be very careful and do my homework, and also have to be very comfortable that the work order is clear. If a misunderstanding happens and the scope is larger than I thought, or if the equivalent of land shifting under a bridge happens to me, forcing a design change, I have to eat the additional cost (or break the contract, if the screwup was of disastrous proportions).

If design changes are required because the customer changes his mind about something, I would get additional payment for that (negotiated as a separate item).

It sounds like in the construction industry there is a bit more leeway for argument. Is that a fair characterization?