Spell Fatigue

[mivayan]

2a) Slaves not only spread fatigue among themselves, they also add to the master's spheres and thus lower overall fatigue.

I dont think higher skill lowers fatigue when communion slaves are involved.

Can anyone explain the following:
An arch theurg with 1 fire magic casts communion master, then fire flies (20 fatigue). He has 8 encumbrance but loses 9 fatigue from the fire flies (why 9 and not 8?). The two communion slaves with 4 encumbrance each loses 18...

Noone is wearing armor, no magic scale in the province.

The only explanation I can think of is that the slaves each get half the spell fatigue and double the encumbrance, is this correct?

If the slaves encumbrance is added, that would explain why they get fatigue when the master casts smite (0 fatigue).