IMO there are two most likely explanations for this:
1: the naval HHQ-9 is shorter than we imagine, or that somehow the naval HHQ-9 is able to be stored in 7m long tubes with its cold launch mechanism
2: all of the 052D's cells are 9m long to accommodate it
I personally think 1 is more likely, though 2 would obviously mean 052D is much more flexible in terms of armament than previously thought.
I don't think #1 is likely, considering HHQ-9 isn't the baseline version of the HQ-9 which is already 6.8 meters. The most common way a missile extends its range is by increasing propellant, and you increase propellant by increasing its length. The Russian S-300 started with the 5V55 which is 7 meters in length. But the next version after that, the 48N6E missile, with a better range, has gone on to 7.5 meters in length.
If they had anticipated increased size of future AAMs, the VLS would have been allowed to have more growth in the future.
As for 054B, my overall argument is not dissimilar -- i.e.: it should have a number of cells that allow it to fire HHQ-9. If that means it needs some 9m cells, then that would work.
My own reasoning is that I don't see it firing the HHQ-9, unless for some reason you want a specific AAW variant of the frigate. The AAW job can be left for the 052D/E and the 055. The 054B should not be a mini, scaled down version of the 052D/E and 055 with the same purpose.
The trend in frigates is not to have large SAMs, but more compact SAMs but with longer and more powerful offensive missiles, cruise missiles or ASMs. The point, if the Type 054B would have 9 meter VLS, is not because of HHQ-9, but because of the YJ-18. An HHQ-9 weighs as much as 1300kg where as the HQ-16 weighs only at around the 700kg. The direction for the 054B should go for is for future ARH type HHQ-16B or a new missile that can be quad packed. The main focus of the 054B should be ASW and ASuW.
Cases in point.
1. The Russian Admiral Grigorovich uses Shtils for its SAMs, which are cousins to the HQ-16. Its use of the larger UKSK VLS is to Kalibr missiles, which has cruise missiles and its antiship missile version.
2. The Russian Admiral Gorshkov uses the Redut missiles for its SAMs, which are namely, short to medium ranged SAMs, and pack the UKSK VLS for the offensively minded Kalibr missiles.
No Russian frigate is armed with the S-300, which would correspond to the HHQ-9. The marine version of the S-300 is the RIF-M, which is fitted only on Kirov class battlecruisers, and on Chinese Type 051C.
3. The RN Type 26 frigates boasts VLS for short ranged SeaCeptor SAMs. Its longer MK. 41, the strike variant, is intended for Tomahawks, LRASMs and ASROCs, not to carry SM-2s, SM-3s or SM-6s. These frigates leave the job of air defense umbrella to the Type 45 destroyers.