Another explanation is that hp and strength (or some other attribute) are not independant: every high hp mage also has high strength, and the so the curve happens to fit well enough by just using one of the attributes.
Hence the dangers of using interpolation for extrapolation
Then again, one could argue that your 1000hp mage with 5str doesn't fit well with dominions.
p.s. I'm not a math genius. I might have it all wrong.