When you practise on your own there is no company address to fall back on, so the address on the engagement letter is the address of your flat. It goes to the client, then to their finance system, then into a procurement record, then into whatever they migrate to next year. None of those people intend you any harm. They simply keep records, as they should.
A home address is personal data about where you sleep, and it behaves differently from a business address once it is in circulation: you cannot recall it, you cannot audit who holds it, and it stays accurate for as long as you live there. Deciding early that only one address circulates is a great deal easier than tracing it after the fact.