A visit to the Sherlock Holmes museum

At 221B Baker Street is a museum devoted to that most famous of amateur detectives, Sherlock Holmes.
Actually, there isn’t, but bear with me.Today there is a 221 Baker Street, but when the novels were written, Sir Arthur Conan Doyle wisely chose a non-existent address, 221 Baker Street — so how is an address that exists today also non-existent?
At the time, Baker Street was shorter than it is today, and what is today 221 Baker Street was at the time 32 Upper Baker Street. The road wa