Severity and relation to occurence - priority?

Question

I have been browsing through some webpages related to testing and found one dealing with the metrics of testing. It says:

The severity level of a defect indicates the potential business impact for the end user (business impact = effect on the end user x frequency of occurrence).

I do not think think this is correct or what am I missing? Usually it is the priority which is the result of such a calculation (severe bug that occurs rarely is still severe but does not have to be fixed immediately). Also from this description, what is the difference between the effect on the end user and business impact?