Originally Posted by FrozenChinchilla
Your calculations in comparing damage were also off.
50 bonus AD is a lot more expensive than 50 bonus AP.
45 AD is 1550
80 AP is 1600
Your calculations assumed equal amounts of bonus AD and AP.
That doesn't change the fact that an AD build is itemizing for AD by level 6, and an AP build is itemizing for Magic Pen. So it's entirely possible to have a scenario in which AD Kat has Boots1, and BF Sword and 20 odd from runes/masteries. AP Kat has Sorcs and a Tome, and whatever AP he has from runes/masteries as well, likely around 50.
It's highly possible that AD Kat has more than 50 bonus AD, while all AP has going for it early game is penetration, and low amounts of AP that items with penetration offer, and whatever he gets from runes/masteries, if any.
Kat has a tendency to need flat defense runes.