Hi Pilates Lover!
I think if a person has strong undertones, some foundations may clash. In my case, I have a light-medium complexion with strong yellow/olive undertones. Color-wise I'd normally go with medium and can handle some peach and some pink, but the amount of beige that works for most medium skin tones makes my skin look somewhat dirty. That seems to happen with those who had stronger undertones. My SIL would look great in medium and she's more neutral with a lean toward olive skin tone. Needless to say, I look sallow easily and when I'm sick, I'm really pale!
I believe the Fair shade is Light on QVC. It can get confusing at times, but it was described as being between the Fairest and Medium. I normally would look too pale in a foundation one up from the most pale! I don't usually burn too easily either unless I'm on certain hormones, The model they chose to represent Fair is pretty pale, I'm not that pale if that helps.
OK, I opened my newer B&B and an older Medium one (don't ask why I sitll have it, I guess so I can talk myself out of ordering it to try for the third time!). The base of both is a flesh-tone color for light skin, withmore shades of browns, concealer-tones, and what would be a medium/tan shade in both. The Medium has more of the darker colors, less of the "plain" light foundation base. The Light (Fair) has thinner amounts of the different shades on the same background, what I think of as a Light powder-colored base that is neutral whereas Medium is definitely more of a mix of colors. They seem neutral to me more than going either way, and I don't really see pink or peach in them as I thought there was before. I likely just confused you more! For me, I think the amount of darker shades in the medium just don't work with my undertones, whereas the lesser dark tones in Fair are something I can work with despite it being more neutral than Golden. Basically, if you saw me and the model for fair, you'd assume she's Fair and I'm Medium. Check out some pics of the actual foundations and you'll see what I'm talking about! 
Good luck, I'll stop confusing you!!! Oh, you might check out the foundation where it's sold alone, the models may be different and it might be easier to choose.