Throw a swing pass to a RB who goes 90 yards, and you look like Johnny Unitas.
Throw a Hail Mary before halftime and get intercepted in the end zone, and get a few drops, and you look like Matt Cassel.
That's not about passer rating -- it's about how tiny samples can mislead. Over the course of a season, and especially multiple seasons, those things even out. That's why the guys with the most top 10 seasons in passer rating are all Hall of Famers.
Peyton Manning (15) 1999-2010, 2012-14
Tom Brady (13) 2001-07, 09-12, 2014-15
Joe Montana (12) 1980-85, 87-90, 93-94
Fran Tarkenton (11) 1964, 67-70, 72-77
Dan Marino (11) 1983-87, 90-92, 94-96
Sammy Baugh (10) 1937, 40-45, 47-49
Johnny Unitas (10) 1956-60, 63-67
Drew Brees (10) 2004-06, 08-09, 11-15
Brett Favre (10) 1992, 94-97, 2001, 03, 04, 07, 09
And no, Jeff George was not a world beater.
Avg rank in passer rating
5 best seasons
Aikman 4th
George 8th
7 best seasons
Aikman 6th
George 11th