Calculation of the living area of a storage room under the stairs

Hello,
can someone tell me how to calculate the living area in a storage room under a staircase?
And what do the two sqm measurements in the picture stand for?
Thank you very much!
Regards
Andreas

Under 2m height half weighted, under 1m height not at all - this difference in factors leads to the discrepancy with the indication of the floor area.

But the 2.91 sqm won't be the entire staircase, will it?

According to the drawing, the storage room is only under the top flight of stairs. That probably fits.