Don't *know* Allan, but have experienced it over time. Just happen to have hit the sweet spot with Inktomi on some sites with the 17.nn% figure, and have just hit a #2 at Yahoo out of 35M with the 16.nn% figure, which almost made me fall over in shock.
There is a difference between the KWD for a 2-word or 3-word phrase and the density for each of the individual words within that phrase, as well as a difference in the weight of the individual words within a phrase. And KWD is not the only factor - it's also the page structure, and how and where on the page the keywords are used. It never does come down to only one thing.
To me, Yahoo is nothing more than the old Inktomi we all know and love, all prettied up with an Estee Lauder beauty makeover - new hair-do and a fancy makeup job. Doesn't get much prettier. ;)