There was a good thread on this, but I'm too lazy and it's too late to link.
The guy above will have little passed balls, little runners try to run on him and when they do, they'll likely get caught near 50 percent of the time. This helps tremendously. The team has another C that caught the other 400 innings and his CERA is only slightly worse. The team also has plus defense defenders at SS, 3b, 2B, CF where defense matters the most and has great pitchers pitching most of the innings. In a park like Seattle, these combinations add up and you can experiment with low PC C's. However, a hitters park, a team with poor defense or a team with poorly rated pitchers would not bode well with a low PC C.