One building, two operating conditions
A flat annual frequency has to be set somewhere between the two conditions, which means it is wrong for most of the year. Set for term time, the scheme overspends through the breaks. Set for the average, the shared areas visibly struggle for forty weeks and look immaculate for twelve. Neither outcome is what anybody would choose if the alternative were put in front of them.
Varying the arrangement with the calendar is straightforward to price and easy for a committee to understand. It also produces a better result for the same total spend in most schemes, because the effort lands when the building is actually carrying people. The saving in quiet periods usually funds a meaningful part of the increase during term. The saving in quiet periods usually funds a meaningful part of the term-time increase.
