If you look at their model vs the correct result, their error appears logarithmic. This is what I would expect from a linear model that is trying to approximate

If you look at their model vs the correct result, their error appears logarithmic. This is what I would expect from a linear model that is trying to approximate a function known to be logarithmic. (The 1-10 PageRank values we see are logarithms of the actual internal Google values, or so it is said.)