Well, there are two problems.
First, to get from Wh to Ah you have to divide by some voltage. What voltage do you use? If you use 3.2V per cell, just know that the answer may be "good enough" for whatever you are doing, but not really accurate since the voltage was changing some throughout the test from 100% to 0% SoC. Like I said, if you are just designing a system, stay with Wh and it will be good enough.
Second, a measurement of Wh is measuring the energy that is going through an entire circuit. So that will include the load are using for the measurement, but also the cables, terminals, and bus bars, etc. These all will have some resistance, and so will be included in the watts you actually take out of the battery, but they will be over and above what you measure.
If you are measuring amps the losses in the circuit don't matter. All of the amps go through the whole circuit, no matter how small the wires or how bad the connections are. So Ah is more repeatable, and can be both more accurate and precise than any measurement of Wh.
As long as you either (1) don't care much about the precision of the measurement, or (2) know exactly what the watt losses are in your measuring system and include that in your Wh calculation, or (3) have low enough watt losses that it doesn't matter you can do just that. Fact is, option #3 is the same as #1 since you will always have some losses.
Maybe I'm being dense, but I'm not sure what you are trying to say here. If you are only saying that 1320 is more than 1280, then obviously I agree. I don't think you really know what the Ah rating is on either of those two Wh measurements, so I don't think the number 100Ah fits.