I would imagine that the increase is due to the extra construction work required for individual showers, rather than the actual use of the showers. Unless the prisoners are usually very friendly in the communal showers. (don't bend over to get the soap)
The report does seem to be rather a waste of time and effort, as I would have thought it was obvious.
I find it rather amusing, a cell with en-suite shower. We are spoiling these prisoners, they wont want to come out.