Open administrator console, go to system panel and open "General Settings". Go to the tab "Application".
There is a subpanel named "Planned Time Calculation Uses:"
From what you wrote I conclude that "Opening Hours Calendar of Assigned Workgroup" is checked. So if the other person belongs to different workgroup with 24x7 calendar the calculations of planned time will be different.
The problem I have is role-specific, not workgroup-specific.
I created a dummy account with the same role but in a different workgroup, and still had the same problem. When I set up the account in the same workgroup but with a different role, it works like it should.
My settings are set for 'Opening hours Calendar of Assigned Workgroup for all items.'