Even if a claim(hypothesis) were well supported...?

Question

by evidence based on good authority, why would ur acceptance be only tentative?

Answer 1

All claims about the observable world are tentative.  Some new observation could change the claim completely.  For example, for most of history a European would have been tentatively correct in saying that all swans are white.  However, when the Europeans reached Australia, they discovered black swans.  Thus, the idea that all swans are white should have been held as tentative.  All good scientists believe that their ideas are tentative.

  This is dramatically different from faith-based systems in which no belief is held to be tentative, for such a view would be seen as lack of faith and, if openly expressed, it might be considered to be blasphemy.

Answer 2

potentially by factors which were unaddressed in the testing of the hypothesis, or by elements which are unexplained. Lots of very good theories can't explain everything. You need to remain open minded about the outcome to prevent yourself from prejudging new evidence or theories which may contradict your position.