What might be going on is that the creatures that can be "killed in the first blow" are low CR creatures (compared to the PC) and the others are higher CR. To be honest, I was never quite sure how Bioware's calculation (in HotU, not the earlier versions) was put together. However, I wouldn't be surprised if the XP award is low if the opponent is easy for the PC and that award goes up somewhat quickly if the opponent is more challenging. Then, whatever that number turns out to be is multiplied by the XP scaling (which one can set in the Toolset in Edit -> Module Properties -> Advanced). If you want the absolute difference in XP awards from easy enemies compared to harder ones to be less dramatic, just lowering that scaling factor may help. Of course, lowering the scaling too much may mean that the PCs don't advance as quickly as you intend as they progress through the module. You can deal with that, to some degree, by giving larger XP awards for non-combat events (quest completion, dialogs, etc.) to make up the difference.
Plenty of modules also have a custom XP system in place. Basically, it's a script that calculates XP for a kill the way you want it and that custom XP award script runs in the NPC's on-death script. By writing your own script that awards XP the way you prefer and then tuning the scaling way down for the built-in XP awards (I think you can set it to 0), you can have pretty much whatever you want.
