I am not Oda so idk. In an SBS he said despite giving him a sword in chapter one he didn't know how he would fight. Here are the facts:
-Shanks before becoming a Yonko was equal to Mihawk
-He then became a Yonko. Note he didn't do it while equal to Mihawk
That strength was clearly not enough
-Yonko Shanks doesn't think he is inferior to Mihawk
-Mihawk doesn't use strength as a reason to not fight Shanks, but the fact that he has one arm
-Mihawk admitted inferiority to deathbed WB
-Meanwhile Shanks clashed equally with WB in WSM condition