In my opinion Law and Luffy are in the same tier or Kizaru and AokijiAs Corazon said above, being in the same tier does not mean your strength is even. If Whitebeard was stronger than Roger, then there strength was along the lines of 100 and 99.9. Whitebeard stalemated Roger, as did Garp and Sengoku, the difference in between the strength of these men are paper thin. Shanks, Kaido, Mihawk, they can all still be in the same tier and the gap can be bigger.
Not only that, but the people you are listing are said to be the last remaining major figures of the previous era that were still active. That's why their names were always mentioned with each other like that. Kaido, Shanks, Mihawk, they're not major figures of the previous Pirate Age, they're major figures of the new, current one, which is why their names aren't lumped in with them.
Notice the phrasings each time were ¨Roger and Garp always cornered each other" or ¨Whitebeard and Roger were fierce rivals¨ or ¨Garp and Sengoku could do this and that." It was always people saying what happened during that age, not them saying that these man stand above each and every single person alive in their own little tier.
People whom if they fight the winner isn't already decided beforehand and it can be 3/10 or 7/10 wins for either
For Roger It will never be like that
He will always come out the victor 10/10 times while in other battles it can vary for him it is indefinitely
I'm not trying to downgrade the admiral I just strongly believe that Roger would always win any fight against them