Head canon based on recent info:
- BB was already very close to WBs level (with his own skill set) when he scarred 24-year old Shanks. He was just smart enough to keep it hidden until the time was right. He kept developing through the years but really not by that much, he was already at a level where he could defeat someone like Green Bull yet he still managed to hide it from WB.
- Shamrock is being over rated similar to Garling who was shown to be inferior by far compared to Xebec. Both (young Garling) and Shamrock might be around 3-4B, i.e. very strong, but compared to those 5B+ guys they come up short. He is not Shanks equal.
- Shanks at 24 is actively holding back. He is at that point already close to that 4.5-5B level but he just shows 98 % of 24-year old Shamrocks level (2B) in order to get intel but not be viewed as a threat. At some point in the flashback he might show his full power. A level which made the likes of the Gorosei, Akainu, Sengoku and Green Bull decide to not go after him/stop fighting.
- Mihawk (and WB) are the only ones (except Kidd who doesn't know better) that don't hesitate fighting Shanks. Mihawk stopped because of honor (although I still think Shanks would give him extreme extreme diff). WB clashed with Shanks (although he probably knew he was too old/sick but still had to put on a face like what Rayleigh did against BB). I suppose this means Mihawk is also under valued, a threat that is best to just leave alone as he until now has been satisfied just targeting swordsmen.