I am not an expert here but it seems dangerous to assume as you do that the baseline of “correct number of ventilators” is the UK number. They could be...

-Under provisioning for surges (not even Covid, even just a sudden emergency of another sort, a bad flu season, big accident, terror event, whatever)

-Going longer between maintenance intervals than is recommended

-Delaying ICU admission longer than is strictly recommended

-Relocating ICU equipment as needed more than other countries feel comfortable doing

-Using more reliable ICU equipment (does the German figure include any older and potentially less reliable gear in the former communist east?)

-Have less need among population for ICU

On the last point, Germany has accepted by far more refugees from MENA war zones than any other European country and I would not be surprised if this resulted in higher emergency medicine use.

Also a quick Google shows Germany is a bit older with 22% of the population over 65 vs 18% in UK.

None of this is to say Germany isn’t over provisioned but it is probably not 5X over provisioned, if it is.