You are absolutely right that such contracts tend to be complicated and to have key aspects that are poorly understood by public officials and their staffs.
In general, governments at all levels do not always understand the terms and conditions of the contracts they enter into with private contractors. The contractors have plenty of opportunity to shape the contract prior to award and they know how to “grow” the contract into even more work.
The government usually goes along with it because they don’t want to get into a work halting fiasco or worse, termination of the contract and the resulting need to go through the solicitation process again. So they usually cave to contractor.
Some years ago, a friend of mine ran for county commission and sharply criticised the bidding and award of a lavish and unnecessary architectural contract to a local firm for work on a new county jail. My friend nearly won the election, but voting irregularities in several precincts and in absentee ballots cost him the election. And his home was burned down while he and his family were at the courthouse waiting on the election returns. The fire was arson, arranged by the local sheriff who got a large slice of the graft on the jail contract, according to a private investigator who checked into the matter.
Washington D.C. is not the only swamp. There are local versions is virtually every state capital, county seat, legislature, courthouse, and city hall.