if we assume the model is correct maybe it’s because differing pressure and bypass ratios ? Plus you have to factor operating RPM and air flow. The GE-F110 at 30,000LBS thrust vs. the F404 17,000LBS. If the F110 is moving more air it will need more fuel. Plus the F-16 seems to cruise more easily it may be at the cost of higher flow rates. Definitely an interesting finding though. I’d have assumed the F-18 would be less efficient do to weight, drag and having 2 moderately powered engines.