Yeah it's not entirely crazy when you are talking about something to quote whatever you can find.
But on this particular index, I strongly suspect it is simply continuing existing biases. It's the thing people quote and by quoting it you get reinforce next year's rankings. Plus it's the kind of thing nobody can ever verify due to its nature.
Assuming what you say here is true (which in my opinion is a pretty high leap), one could then ask why those biases exist in the first place. I expect authoritarian states to enable corruption but it's the contrary in the case of Singapore. Why? Because I doubt it boils down to the salary of the ministers.
On a side note, it might be interesting to see the evolution of this index over the years for any given country; that might lead to interesting conclusions on the bias confirmation you're talking about!
But on this particular index, I strongly suspect it is simply continuing existing biases. It's the thing people quote and by quoting it you get reinforce next year's rankings. Plus it's the kind of thing nobody can ever verify due to its nature.