I agree with you that the low numbers being put out are for public consumption and may not represent the true number, if you could financially audit them in completion for their entire history.
The finances of the British monarchy work like the Mafia’s Las Vegas casinos did back in the day. On the books, the assets are valuable and always do well, but the real money was in the unreported skim or, with the monarchy, in the steady accumulations of reported and unreported income — and how much that is and where it all goes, no one one on the outside ever fully gets to know.