Why does my total heat loss not equal the sum total of the room heat loss?

If you are using one of the BS EN 12831-1:2017 methods, the ventilation losses for a building are not a simple sum of the ventilation losses for each room. Instead ventilation losses are calculated both at a room level and then separately on a building (or zone) level.

On the building level, there's an additional factor (f_i-z), typically assumed to be 0.5, which makes the building ventilation losses smaller. Read more about this here.