In 2011, the state of Illinois increased its state income tax rate from 3% to 5%. One could communicate this by saying that people will be paying 2% more of their incomes to state taxes which doesn’t sound all that bad since Illinois increased its rate rate 2%. Or you could say that Illinois increased its state income tax by 66% which sounds catastrophic and is also correct because you’re changing what the number actually is referring to. Increasing from 3 to 5 is an increase of 66% (or 66.6% to be more accurate with a bar over the last 6 but I don’t know how to do that on a keyboard). People would react to the news differently depending on how you communicate even though both could be considered correct.
Here are two different sites reporting the story. Notice how the choice of language and numbers changes the feel of each story even though they are both reporting the same news.