Is this not a function of how rationality actually works? We all begin with premises which are fundamentally unproven and unprovable. Our conclusions follow from those premises. Change the premises and the result changes, though both sets of deductions may be perfectly rational. In other words, rationality is a process, not a set of conclusions.

One word for this is "tradition". If I've piqued your interest, please read Alasdair MacIntyre's book "After Virtue," and also it's sequel with a more provocative title, "Whose Justice? Which Rationality?"