in saturation of BJT, the current gain is reduced
I think you are asking how beta can be less in saturation than in active mode when it appears from a calculation of Ic that Ic is highest in saturation.  If that is your question, the answer is that in saturation, if you increase the base current this fails to further increase the collector current. So in betasat = Ic/Ib, Ib increases but Ic does not, hence betasat reduces.