When you are building more than one building at a time, the time shown for the consecutive buildings to finish will be much longer.  This is due to the fact that the buildings to be completed, will be in a queue and will display an actual time left to build them (including the finishing of other buildings in the queue).

So for example when you upgrade 3 buildings, and all of them have a building time of 1 hour 1st building will finish after 1 hour, 2nd building after 2 hours and 3rd after 3 hours. The times displayed will also be  respectively, 1, 2 and 3 hours despite the fact that each one has a building time of 1hour. You can speed up all buildings in the queue but you can only finish them chronologically. This means that the first building you put in the queue is the only one that can be instantly finished, after it has been, you can finish the second one, and so on.

The same queue system applies to recruiting units (with the exception of the queue being free for up to 3 simultaneous units, including transport, i.e. market units).