In my experience ITA is a little too keen to apply circle trip minimums for more complicated routings and more so on certain airlines (LH being the worst). When it comes to ticketing, these extras often disappear. This makes the fares ITA yields that much more complicated to figure out as more often than not, you end up paying less in real life.

NUC btw stands for "net unit of construction" which is an old IATA attempt to standardise to a world-wide currency. I am not sure how you work back to real world currency values from the NUC but AFAIK IATA releases (or used to) a standard exchange rate at regular intervals (perhaps once a year?)