It's also a good example of why no one goes high when it comes to estimates.
The realized cost is now to a point where it'd be politically unpalatable to approve the project. But since it's already in process, approving the incremental funds is essentially inevitable and won't cause much political backlash.
These estimates aren't low by accident. They're generally as optimistic as possible while still being in the realm of marginally plausible (even if wildly improbable). Once the ball gets approved and rolling, the momentum alone is usually enough to hurdle past objections as estimates meet reality, while diffusing responsibility (and therefore repercussions) along the way.
