As you can imagine, there are unlimited variations on how construction contracts, particularly large contracts with obvious schedule drop dead dates (Start of a season for example.), are written. My experience having written and negotiated quite a few (mostly hotels) is that contractors perform far better when there is a bonus structure (There are always penalties/liquidated damages provisions.). One of my most successful projects was one where I included a $1 mil "no fault" bonus provision for meeting a date certain (With defined criteria.). The contractor was free to accelerate work or not and the typical delay causes were irrelevant (Weather, force majeure, owner and his contractors (Architect for example.) caused delays, etc.). Opened on time, on budget on a project that in 2000 had a $350 mil construction budget and a $600 mil overall budget. I know the lawyers here may parse how to write such a contract but we did. Don't remember details now eight years into retirement.
The point is NU likely has some form of bonus/liquidated damage form contract with fixed allowances for weather, contractor contingency budgets, etc. Quite possibly the contract is more creative as this isn't NU's or the contractor's first rodeo. Public contracts typically have more difficulty writing these types of contracts but certainly not entirely. There have been examples of very creative and successful public contracting approaches. The public typically isn't as aware of the successes vs the disasters.