Because being balanced for Random Battles and balanced for competitive are two different things.
Petropavlovsk was not the terror in randoms that it was in Competitive because a team of players working together with a predetermined plan and audible coms can make maximal use of what Petro did best - tank and deny caps with radar at minimal risk.
Ships are balanced for 12 v 12 inherently, so when the game modes go to 7v7 or 9v9, certain ships begin to shine where they didn’t so much before.