It's mostly due to the anime, where it's potrayed as whitebeard not being able to use his full strength yet mopping akainu who was the strongest admiral and making akainu look scared.I mean all together. I believe all Yonkou and Admirals are about the same anyways.
The highest calcs Whitebeard heard, were ironically matches and stopped by Aokiji and Akainu.
People have this weird obsession over Old Beard being the strongest. And it's just....weird?
Also in the anime whitebeard doesn't lose half of his face which skews perception on him.