I added some pages from the Bentley regarding the topic, i.e. manual gearbox (01E) and fluid level/check considering front/center diff.

So the answer to the OP question (capacity for gearbox/front/center diff) is:
3.8 qt.

Any answer available why he could to get even close to that into the gearbox?

I have given it a lot of thought and concluded that I should have disconnected the gearbox oil cooler sending line and flushed it out with air (or disconnected the return line and vacuumed it out). I am betting that about a quart sits in the cooler and can not drain out on its own. Just have a crush washer on hand to put that line back on.