This is not a unique "problem" to cars only. The same is true in central ventilation in buildings aswell.
When the pipes to two or more rooms (or vents in this case) is not the exacte length and have the exacte the same bends, there will be a difference in the airflow.
This is why a ventilation expert will tune all the vents in a house so all blow the same. This is done by opening some vents fully and shutting others partially.

You can do this yourself!
The easy fix to get all vents to blow the same is to close the centeral vent partially. You will NOT get less air into the cabin (this amount of air is dependent on the fan speed).
What you will get is a little less air in the centeral vent, but MORE air in the other vents.
If you want to maintain the same airflow in the central vent, increase the fan speed a little to maintain the airflow.
Now all you vents will be the same!

For those that think BMW should have made sure all vents are 100% the same, think of it the other way; you would instead be pissed that BMW restricted the possible airflow in the central vents those times you wanted more air there!

So, adjust your central vent and be happy! That is why they can be adjusted.