I can't say what's causing the difference in your case, but I can tell you that often what wee see is a computational error somewhere, in Excel if that's what someone is using, or in SPSS formulas or whatever. If you've built a model with interactions. Even more often its an order of operations issue where someone computes importances from averaged utilities rather than averaging importances from respondent-level utilities. On occasion I've seen folks compare importances calculated from one model (e.g. HB) with those our software compares for another model (e.g. MNL). I've seen a lot of creative variations, but I can't be sure which applies to your situation. It sounds like you're doing it right, but if so I'd expect the importances to match exactly.