How To Measure Rosh Hashanah Services

My wife and I were speculating on how long last Friday’s Rosh Hashanah service would last. We both figured on two hours, but my wife said, “Services always last longer than you expect.”
My first reaction was to agree, but then I realized that couldn’t be so; it would imply that I didn’t have rational expectations. Having attended services for so many years, my overestimates and underestimates of their duration should balance out: on average, I should correctly estimate their duration.
It’s possible that I might make mistakes if the world changed; and perhaps our new rabbi goes longer than his predecessor. But even with that change, after a few years I should estimate correctly on average. In fact, taking Friday and Saturday together, I was correct: Friday night lasted only one and a half hours, but Saturday’s service lasted half an hour longer than I expected.