Don't worry at all about asking silly questions!
In that example the Devanu selected to cast 4 Erac and 2 Oran, but when he cast them he only managed to get 1 successful oran and 1 successful erac (see the diagram, it isn't explicitly stated in the text). The Fubarnii cast 4 erac, and managed 2 successes. At that stage the Devanu's oran would cancel one of the Fubarnii's erac, so both sides would land a single blow. Hopefully the rest of the example makes sense from there in, but feel free to ask if you need any more clarification.