I've seen this a couple of times, looks a little odd ;)
Also, when done with the build, the duration is still different, but in a different way:
Edit 2: the last is the combined time of all subbuilds, as if they didn't run in parallel. So that's at least understandable :)
Looks like a small js issue as you will notice in the first pic that it is just 1 hour greater than it should be.
Pull requests accepted :)
This is probably due to the way duration is calculated. Duration is (roughly) the time between the first job in a build matrix is started and the last job is finished. Depending on how congested the queues are, the duration can be quite large. If there is something else to be looked at, we can investigate.
For now, I'm closing this.