35d transmission fluid - 4 litres out 2.5 goes in

[V12noise]

Did a gearbox oil drain, sump change and fill in my 2010 35d. Measured 4 litres out. Changed the sump, refilled to the fill hole which took 2.5 litres, ran it through the gears N-R-1-2. Could only put a few hundred millilitres in. Thought it strange.

Took it for a slow drive using all 6 of the gears, and still at the same level, i.e. oil was coming out the fill hole.

So have I done something wrong or do I assume it was overfilled by the previous owner/ service-agent? Looking at some YouTube vids, 2.5 litres seems to be a reasonable amount...