It's entirely possible that despite being Oda's golden child, Shanks is, in Oda's mind, weaker than Mihawk. After all, it's entirely possible that Shanks, despite all that hype, is still weaker than Kaido, the WSC. Being Oda's golden child just means Shanks will have way more impact in the story than Mihawk, and their strength is relative, with Shanks being slightly weaker.
After all, the only time where Oda outright compares Mihawk to Shanks, Oda puts Mihawk ahead. Shit I think that's the only time where Oda outright compares a top tier to one another through statements.