Where do you get that number? All the statistics I've seen from dating sites indicate that while women have an 80% cut off men have a 50% cut off (i.e. truly average).
In other words, while women consider any men below the 80th percentile "below average", men's opinions reflect the actual average (i.e. considering any women below the 50th percentile "below average" and above it "above average") rather than (as you seem to say) considering any woman above the 20th percentile "above average".