I tried running the same scenarios in ALOHA and in RMP*Comp. I got different answers. Why?

The results you obtain using RMP*Comp may not closely match the results you obtain running the same release scenario in a more sophisticated air dispersion model such as ALOHA or DEGADIS. That's because of a fundamental difference in purpose between those models and RMP*Comp. RMP*Comp is a planning tool, designed to help you to easily identify high-priority hazards at your facility. It makes simple, generalized calculations. In contrast, models like ALOHA and DEGADIS are intended to give you as accurate an estimate as possible of the extent and location of the area that might be placed at risk by a particular chemical release. They account for many more of the factors that influence the dispersion of a hazardous chemical. (For this reason, when you need to make decisions during an actual response, use models like ALOHA or DEGADIS, not RMP*Comp.)