Hello Steve,
I can speak for the FlowMaster side of things - generally speaking the inlet calculations in FlowMaster are indeed based on HEC-22. However, many of the HEC-22 inlet equations leave some openness to interpretation. So, FlowMaster (and StormCAD, which is the basis for hydraulic calculations in Subsurface Utilities / OpenRoads Designer) has some enhancements on top of the HEC-22 standards for certain conditions, which attempt to provide more appropriate results. In some cases the differences are in extreme conditions like very long curb opening length, but can also come into play when certain things are involved such as clogging and local depressions.
If this is for a grate inlet in sag, the following article in the Haestad Wiki should be of interest: