Redundancy. It was common in the Russian/Soviet navy to have a backup program incase one didn't perform as the designers said it would. Thats why the Russians have so many diffrent classes of ship.
For example the Slava class cruisers were a backup incase the Kirov didn't prove practical. The Slava was a conventional (for the Russians) design while the Kirov was more cutting edge. Or the Sierra and Akula, the Akula was a consertive backup when it became obvous that the Seirra was going to cost way too much.
BTW I've seen a 67 knot max speed for the 53-65K.
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