I hesitate to dig deep into these numbers (and trust you have) but.....doesn't catcher era include more than just the pitcher era? there's adjustments based on the quality of the of the pitching staff plus maybe some other variables ?
from the Fielding Bible
How do you measure Adjusted Earned Runs Saved?
Much more than simple Catcher ERA, Adjusted Earned Runs Saved accounts for the quality of the pitching staff the catcher works with. There are still many other variables in play and some noise in the results, so we regress the final data to a more reasonable scale.