Yep, as the poster above me said, MAC do it the opposite way round to basically everyone, it goes against what I was taught when I was training as a make up artist
NW = neutral warm (pink undertone)
NC = neutral cool (yellow tone)
I've also been told to think about it like NW means 'Needs Warmth' and NC as 'Needs Cool' but I think that's a bit odd...
The issue I have with MAC foundation is that a '20' level is meant to be the same shade through the whole range, but they do change based on formula, some run darker or lighter than others, and some are more prone to oxidising and going dark than other (Studio Fix Fluid being the worst offender of this) it also annoys me that in general, at the lighter end of the shade range, NC colours tend to be lighter than their NW counter parts (Studio sculpt, lucky for me is about two shades lighter in NC than in NW)
I'm very pale, the only MAC foundation colour that works for me is Studio Sculpt in NC15, which is supposed to be a yellow toned shade but it actually more neutral, my undertone is neutral leaning slightly towards cool but for the most part I have no strong undertones I'd be the perfect N10 if they made that shade *shakes fist* for reference I'm 115 in Illamasqua Rich Liquid, 02 in Illamasqua Skinbase, Alabaster in Bobbi Brown (lets not discuss their whole 'we are all yellow philosophy') and off the charts in most brands :P