Sneak attack questions

So I'm using sneak attack on my mage and notice I have 10 charges so when I cast impact finale it usually consumes two charges which makes sense but sometimes it consumes 3 charges and when I was at venena now my impact finale consumes 5 charges and sometimes 7.

Why do I sometimes consume more charges than other times?
I'm using combo ad-lib>sneak(save) >prayer(swift), i use that buff in beginning and then cast finale and as i said usually only two or three charges are taken from sneak but now at venena its 5 and 7.
What am i doing wrong ._. Annoying my sneak gets used so fast.

 

It costs 1 charge per mob hit.
If there are balls around, each ball increases its cost by 1

 

Alright makes sense. Thank you

 

One more reason to kill these sneaky coenubia balls