Assumptions in financial performance analysis, allowed or not?

I would not postulate causes or make assumptions without some evidence from the question (unless you seem to be required to do that). It would be be better to say that it is not clear why income has increased, but be sure there are no clues in the question.

S/D 2020 If the CFO does not know the cause of cost changes then the CFO is a fool and using the word 'may' simply draws attention to that. Better to say that the causes still need to be determined.