Ok, so here is what I have discovered (This is new to me too, it was inherited from version 2.45 "If it ain't broke don't fix it")
The first interesting thing is you can make or unmake a max of 100 units of slime per cycle. This means if you are storing more into slime it will be capped at 100 for you.
The second thing I have discovered: (and I might have seen this before but never cared enough to remember in detail)
There is a "fixed" cost for slime on top of the normal costs. It turns out that slime costs 1/10 energy per slime.
Therefore, Your robot is actually producing 100 slime per cycle and gets charged 10 energy points each time.