Then again, the two year extra salary cost as a percentage of the contract cost is probably below rounding error.
Anyone have an idea why or how it coild be fixed?
When you hear about massive projects being done cheaper (especially in per-mile costs), often it's because the project isn't running through an established, densely-populated area. So what you end up getting is "cost of building in a wilderness" versus "cost of building in a dense established city". The latter is going to be more expensive.
Not to say there aren't valid reasons for some projects to be more expensive (especially when you're tunneling), but by all accounts the US manages to have outsized costs for pretty much any infrastructure project.