In each Clip-thumbnail there is showed a time, like in this one, which says 7h26m57s. What does this mean exactly ? This clip does not consist of 7h26m video recording, so it must mean something else I guess.
Keep in mind a clip is not an alert. Alerts are triggered events, usually motion, and typically a few seconds but can run much longer depending on the duration of the trigger event. A clip is a section of continuous recording and does contain the alert video as well. Alerts are just "location markers" in a continuous record file.
I think it makes sense now. If there is a recording of like 10 sec at 1 AM and then again another recording of 10 sec at 5 AM, it would show 4 hours, but only consist of 20 sec of recording...I think.