If I am not mistaken, the range is when the aircraft is flying
in L/D Max.
That is, high altitude for low drag, throttled to most km per lb of
fuel burnt.
More weapons, more drag+weight, lesser range.
It doesn't mean the the F-15 is inferior if its range is measured
in such "optimal" conditions, without combat load. For another
aircraft to carry the same amount of combat load, it would probably
have half the F-15E's combat range with the same load.
