Agreed. I for one feel that it’s not very important, and I wouldn’t implement a monarchy if designing a country from scratch, but I also wouldn’t remove the current one.
To me, a newspaper combining those groups says "most Britons in favour of a monarchy with some power, but we don't want to admit that" more than anything.
People who say they are indifferent do not support the Monarchy and just live with it. So there is a reasonable argument for combining those categories of your aim is to sample support.
Ask Americans or the French what they think of their republic and I doubt 45% will be against or indifferent.