I dont' think it's so much about each navy's "current" needs but rather a combination of current/foreseeable needs along with future proofing.
Having a longer VLS means you can also put in a longer missile in future as they're developed.
However, it's also worth nothing the Chinese Navy's VLS is a type whereby their missiles are cold launched or CCL hot launched, and in both cases, a small part of the overall "length" will be taken up by the "launch mechanism" which in the case of cold launch will be the compressed gas ejection system beneath the missile, and in the case of CCL hot launch will be the bit of venting that will likely be integrated to the canister below the missile.
In both cases, it means that the actual "useable" length of the missile is actually a little bit less than 9m due to he associated apparatus with both launch mechanisms
