This is an oddity that you could see in regular sins with multiple factories. When producing multiple units, the calculation is correct in that you can build as many units in parallel as you have factories, but which factory is building which unit will appear to change when one unit completes. If you look carefully, the "progress" of a unit will appear to waterfall down from factory to factory until it completes. This also means that if you watch one factory after queing multiple items it will appear to reset construction. This also means that I can have three factories but the output of the three factories physically exit one 'favorite' factory.

It seems that factories are assigned an order, and that all build orders issued to the planet are allowed to complete by a factory in a predictable, deterministic, way. If I had to venture a guess, it's done this way to prevent a multiplayer sync issue without excessive extra communication.