On the trakt website, when displaying the collection view, displaying shows and ordered by runtime or number of episodes, the displayed runtime is not correct. Sometimes is only a few hours off, but sometime is completely wrong… You can see this for 3 examples in the screenshot. I displayed the correct “Watched” summaries below it.
How are you adding the plays? If it’s scrobbling by media center, we use the runtime reported by the player so that could account for the difference in runtimes.