The article suggests that it’s a feature of the way we perceive colour: if you get a spinning top with half blue and half white and spin it so the colours blend together (ie taking the average of the spectrum from each colour), it will shift towards purple as well as lighter. To get a gradient that seems to go from blue to white without going via purple, a colour space needs to counteract this aspect of our vision.
I was noticing that as well. They seem quite off in a surprising way