Thank you for that question.
The definitions that are used in SOIA are with the context of SOIA and the offence itself in mind and were clearly inspired, as you note, by other provisions and other categories of public office holders, so the offence of interference with political processes or governance involves a category of public office holder that I agree is quite extensive. In fact, as it's defined, it is open-ended to a certain extent. That works well within the context of SOIA and within the context of that particular provision.
The other categories of public office holder are designed to work well with the other regime. That's what I would say. Yes, there are similarities and yes, there are differences, but those are intentional.