Well commercial galleries shouldn't be there as their artworks will move once sold.
On the other hand, Burghley House owns some artworks for centuries. The problem with these "museums" is that their catalogues aren't always up to date. So we never know if an artwork is still located there since they sometimes "discretely" sell some of their belongings through private transactions, whereas official museums are usually not allowed to do so. For example, even for a famous collection such as the Royal collection, there is always a doubt on the location of an artwork (is it in Buckingham or Hampdon Court?...)
In respect of the Royal Collection, it may have been best to have had one museum entry - 'Royal Collection Trust' but this would remain ambiguous (not that that matters as the majority of works are not on public display and are 'private').
I have tried to ascertain where each artwork is located but the Royal Collection website (which is excellent) doesn't usually state the whereabouts - that is probably due to the collection being moved from one property to another or to the Queen's Gallery, London, where public exhibitions are held.
A few works are permanently held in a particular property (Hampton Court Palace, St James's Palace and the Palace of Holyroodhouse). The major dilemma on location is between Buckingham Palace and Windsor.