Capn Bloggs
You may have overlooked it, but In my post I included the phrase (But by no means the longest possible explanation).
Yes of course we can go into the added complications of Varying specific fuel consumption and also varying propulsive efficiency. But Balderax's post ruled out engine performance and the tone of the question suggested (to me at least) that he/she was looking for the most basic analysis.
By all means give us an explanation which encompasses all of the possible factors, but I suspect that this is not what he/she was looking for. But every day should be a day in school, so please go for it.