The total residential population of a structure is the number of persons living in that structure at one time, regardless of whether they are at home or not. This is an actual number rather than a potential number; no persons are counted for any vacant unit.

A population is a measurement taken at a point in time, so a figure is only usable with the date it was counted on and the source it came from. Two figures for the same building in different years are both correct.

It is not a capacity. The full capacity is what the building may legally hold and the population is who is normally in it, so the population is smaller and the two answer different questions.

The residential and staff figures count different people and overlap only where someone lives and works in the same building, so adding them describes nobody.