When you're clicking through the effects list, looking at the timeline, you'll find that all the durations don't show at once. That's because durations show in time-related groups. That is to say, effects whose start depends on the same effect (say, one that plays on display of the slide) show their durations together. The durations show up when you click any effect in the group.
Whenever there's an effect that breaks up that time dependency, for example one that starts by mouse click and so isn't dependent on any effect preceding it, a new series of timings is introduced. Such an effect "resets" the clock for the slide: the timings start at zero again because you can't time when the mouse would be clicked. Durations for any effects that follow the mouse-click effect and are dependent on it would show as a group in the timeline.