AGi32 and ElumTools both use the same calculation engine. The results will be the same if they are given the same input.
The most likely reason for this discrepancy is the reflectance values in ElumTools and AGi32 are different. ElumTools will automatically apply a reflectance value based on the material of the building in Revit. Sometimes though this can result in very high, sometimes unrealistic (Over 90%) reflectance values that are much higher or lower than the default values in AGi32.
You can check the reflectance values by opening the Material Mapping option in the ElumTools ribbon.
A good way to compare ElumTools and AGi32 more accurately would be to use the Category Overrides to change the reflectance values of all walls, floors, and ceilings to match what you have set in AGi32. By default these are .80 (80%) for Ceilings, .50 (50%) for Walls, and .20 (20%) for Floors. Check the box by Category and select Ok. ElumTools will now calculate with those materials overridden and the results will most likely be much closer to AGi32.
In most cases as well, the AGi32 model and the model in Revit do not match completely. Modeling differences could also explain some of the discrepancy.