The article is a bit sketchy on the methodology used. The fact that not everybody flat investigated has somebody on the electoral register can easily be explained by several factors.
- The central London population is very transient. Many stay only for a few weeks, months or at most a year or two. Why bother registering?
- Many central Londoners, especially the young and newly arrived, move about quite a lot before they settle in one place for longer. They would not bother with registration.
- Many, probably the majority of central Londoners are foreign and don't bother with the electoral register because they can't vote in the general elections anyway, and may not know that the can vote for the major (or may not care).
- There's a lot of legally questionable subletting going on (think AirBnB), where the subletter and / or sublettee might not necessarily want to be on an official register.
- Some owners may simply fail to find a tenant, for example because they pitch rent too high.
As a piece of anecdotal evidence, I offer myself: I only got on the electoral register about 10 years after I moved to London.