Reasons, resultance and moral particularism

According to Jonathan Dancy's moral particularism, the way in which a given moral reason functions as a reason for or against an action can vary from case to case. Dancy also asserts that reasons are resultance bases. But a reason why something ought to be done is that in virtue of which it is something that ought to be done. If the function of a reason can vary, then resultance bases cannot be reasons. Perhaps the particularist might conceive a reason not as a resultance base, but as a specific type of which a resultance base is a token. But this picture of reasons cannot be correct.