Since the difficulty is the proportion to the amount of time over or under two weeks the previous 2016 blocks took to find, it's a deterministic calculation tha

Since the difficulty is the proportion to the amount of time over or under two weeks the previous 2016 blocks took to find, it's a deterministic calculation that other nodes use to accept and validate each new block. And since it's in the best interest of nodes to keep the longest chain of blocks, it's not in their interest to break this rule. (or they will end up on an abandoned blockchain fork)