I noticed in my schedule, I have columns that have a negative volume:
When looking at the buidling Element Info from these colums, it seems to be a positive value:
When manipulating the column (even just 'moving' the end point to it's same position, so no real change happens to the column) the value in the schedule turns positive.
This is dangerous when using a pivot table like I do to give you the sum of all column volumes, since a negative one obviously is subtracted instead of added.