I think this notice period is a hangover from the old legislation ( particularly in NSW). IF a contract is due to expire on a date, it expires on that date. Irrespective of whether the contract says a notice period is required, without an renewal of the contract by the Owners Corporation, the services of the strata manager is terminated on that date.
If the strata manager was to continue with their services past the contract end date, and without a renewal, you would have effectively an unexpiring contract, which modern strata legislation aims to avoid.