Because four of the six NFC wins are against weak AFC teams, including two wins against Houston (1-3), one against Tennessee (0-4), and one against Cleveland (1-3).
The combined record of these AFC teams is 7-16, 30.4% winning percentage. In other words, these AFC teams' combined conference record is only 7-10 (41%)
Although AFC also beat some weak NFC teams, the combined record of the NFC teams losing to AFC is 6-11, or 35.3%. These NFC teams' combined conference record is 6-6 (50%).