We do not know why because we do not know what is in your composition between each of the time ranges. Did you know that not everything in Shotcut runs at the same speed all of the time? I would have thought that is obvious to anyone who has used it more than a few hours. We also do not know what else the computer was doing during each of these. Another answer is: it depends on many things.
is it possible to get the faster times also for the other exports?
I doubt it, but you can simply try some of those more than once to see if you get the same result.
Maybe the edits in the timeline during the last 3 markers used fewer filters and transitions. That would explain the time difference. Some filters are considerably slower than others. For example, Size, Position and Rotate can considerably slow down export.