Max panel string voltage - STC or NOCT values?

Wholesale Solar has a guide for string sizing here.

In it, they say something that surprised me. They say, in order to calculate the max string output, you should:

1) Take the STC Voc (eg. the Voc measured at 25C), 48.08V in their example above
2) Take the temperature at NOCT, 44C for their example panel
3) Find the delta from the NOCT temperature and your minimum design temperature, 77.3C in their example
4) Use the panels Voc temp coefficient to derate the STC Voc by that delta

I guess my question is obvious:Why are they using the NOCT temperature (44C) but the STC voltage? I would have thought if you were derating the STC voltage you'd "start" at 25C, the STC temp, rather than 44C?