Hi Russell - what's 'listed' when you search is address information/data only based on your own search criteria. It's not 100% the registered information so you can't conclude from the listing that the 'missing' garages are not registered.
If you download the freehold register then that should refer to each registered lease be they be of the flats or flats and garages as appropriate. And assuming there's no head leasehold title involved.
However if your focus is on the garages then I would recommend doing a
postal search of the index map instead. We do the search and provide a result which confirms the title numbers for the land (garages) as appropriate