You're quite right - I too won this on the first attempt, while there are others with 'easier' ratings that pose a much greater challenge.
The decision on the rating is subjective. You are more likely to see 'oddities' in the difficulty ratings of enthusiast-authored scenarios than the ones produced by the companies, as the extent of playtesting (you would expect) would usually be more thorough with the official scenarios and therefore the (subjective) rating 'closer to the mark'. In this case, I think the rating was overstated.
In answer to your question - "computer programming experts". Holding <Ctrl> and typing rateddifficulty
when wishing to edit the scenario allows you to change the difficulty level (as identified by the scenario's introductory information).
Hope that helps.