Overthought a bit, i can imagine Marco having a higher Bounty than King, because it is About the threat the Person brings to the WG. Whitebeard obviously knew some stuff about the One Piece, Maybe the Location, Maybe even the history himself. The WG does not know what exactly whitebeard (and with whitebeard even Marco) knew About the one piece and the history. So if they Need to assume a possibility of Marco knowing the true history and the location of the one piece, he will have a high high threatening Level, even tho he Maybe doesnt know anything.