Vas - Scan Speak 15W8530 K00

I am a little bit confused. Despite a long play-in, the driver still has a very low Vas of 10-11 Liters.
According to Scan Speak data, Vas is specified with 16 Liter.
(fs ... 39Hz specified ... 48Hz measured)

Has anyone a clue if this is a common divergence?
I am asking this, because the same model, 10yrs ago, had different TSP, but measurement was nearly equal to manufacturers data.

Many thanks