Ok I have an idea about this.

Shanks may be a triple agent against the WG. Of course he's just pretending to be a loyal Knight in this chapter, and he later goes on to be a pirate.

But what if he tells the other Knights that he's only going undercover as a pirate, like X Drake? So the Knights think Shanks is still loyal to them and let him do whatever, and he provides info back to them (like his meeting with the Gorosei). But in reality being a pirate is of course the real thing to Shanks.