Yonko want to take down government and government want to take Yonko down. So, Gorosei and Shanks are direct opposition and at seeing each other they should clearly start a battle or something like that. But surprisingly nothing of that sort happened and this indeed is very suspicious. Why was Shanks allowed to enter Mariejois? How is that even possible?
Gorosei treated Shanks with respect instead of anger. This raises a lot of eyebrows. And the only way I think that this is possible is that Shanks has Celestial Dragon lineage. That is why Gorosei made out time from their busy schedule to set a meeting up with Yonko Shanks. This will be nothing new for us as we have already seen this sort of thing happening in the past. We saw Doflamingo aka Joker who is a well-known pirate and was a Warlord too. He has the Celestial Dragon lineage as well, and since he knew about the National Treasure of Mariejois, he could even control the CP0.