There is a little bug when spores hits pools of creep with more creep in the pool than in the spore.
I explain : if you have a pool filled at 80% of the max heigth of the map, then a spore with 20% of max heigth (depends of intensity),
then, the pool, on the pixel where the spore landed goes to 20% then it refills up (slowing the creep advance)
It means that the creep contained in the spore replace the one there was, instead of adding to it.
Well.... sure enough. How about that. Never noticed.
I'll put this fix in the next update (which is probably coming this weekend).