Sorry to hear of your refusal rhaichard.
The problem is, you're dealing with government officials who are working to strict rules, then just ticking boxes.
They are evidently looking for a 'guaranteed' minimum income level.
Your partner simply needs to be able to show...on paper, a minimum basic salary of £18,600 per year.
I think that would have been clear to most people.
Personally I would have contacted the UKBA for clarification before submitting papers, and I would urge anyone else to do that too, if in any doubt.