Float at WBS Level / How Float is Calculated at WBS Level

How in all of the God's names is the 'Total Float' value shown at the WBS level calculated ?

For some WBS levels, the float appears to be the lowest (most critical) float value of the activities contained underthat WBS level.

However for other WBS levels, the float mayto be the highest (least critical) float value of the activities contained underthat WBS level , or even something in between.

I can not understand how the float value at the WBS level is calculated - can anyone help undersand this??????

For background, I am trying to share a 'rolled up' part of a program with my team, showing only WBS levels rather than hundreds of underlying activities. But they also need to see the lowest / most critcal float values for a WBS.

Thanks !