I totally agree. It is impossible to compare two completely different aircraft in terms of the engines they use, the number of seats they have, their likely cruise altitude, as well as whatever extra weight they were carrying on the route. In addition their route length may have been slightly different depending on the approach procedure they used (the 738 could have used RNP approach for example).

As Adam Dowley says, the two calculations are completely different so of course the answers will be different. And the fact that the 757 has a higher number of seats will mean that you have divided the fuel burn down by more anyway.

Indeed the best idea would be to look at the lb per hour fuel burn for the aircraft as a whole, then you will see that the 738 uses a lot less fuel than the 757.