The problem is simply that their downsampling is not gamma-corrected, so overall brightness is not preserved.
https://erikmcclure.com/blog/everyone-does-srgb-wrong-becaus...
linear-gradient(in srgb-linear, ...)
Works in Safari/Chrome but not yet on Firefox
Not sure if it’s still up to date, so please check out the list of resources mentioned in the footer too.
Happy to report that in the last few years since 2007, GIMP did fix their default colorspace, I can scale the Dalai Lama down even with the Linear filter and it doesn't turn grey.