Because the Royals choose to obfuscate how much the most high profile members of the family cost.
I'd like to be able to claim they cost £5 million per year but I can't because some of that money goes towards machines to cut grass.
Bear with me and I will explain.
Understanding Royal Finances is a complex business.
It's partly because the money which pays for the Family's activities and duties comes from many different pots.
There is the core Sovereign Grant - 15% of the Crown Estate revenues which would otherwise be returned to the treasury (hence why it's called "taxpayers money").
You also have the "additional" Sovereign Grant - an extra 10% of Crown Estate revenues for a period of ten years to pay for the programme of repairs at Buckingham Palace (officially called the "Reservicing Project").
The Queen has a private income from the Duchy of Lancaster on which she pays some tax.
The Royal Collection Trust collects money from visitors and tourists to palaces like Windsor Castle or Holyroodhouse in Edinburgh.
And the Duchy of Cornwall pays most of the work of the Heir to the Throne - currently Prince Charles - and his children.
The Duchy of Cornwall is a private income stream and is not, as the Prince of Wales' office insists, taxpayers' money.
It generated an income of £21.6 million in 2018-19.
Last year, it funded 638 engagements for the Prince of Wales and Duchess of Cornwall in 14 countries.
The work (but not the travel) of the Duke and Duchess of Cambridge and the Duke and Duchess of Sussex comes from that pot.
It would be easy for the Royal Family to reveal exactly how much the Cambridges and Sussexes cost.
But they choose not to.
And - as we discussed above - this is a private income stream, not taxpayers' money, so Prince Charles' office cannot be forced to reveal it.
The box in which the Palace details the costs of Prince Charles' sons and their families (so William, Kate, George, Charlotte, Louis, Harry, Meghan and Archie) is not straightforward.
In 2018-19, it cost £5.1 million for what it describes as "funding the activities of The Duke and Duchess of Cambridge and The Duke and Duchess of Sussex".
So far so simple.
But the £5.1m also includes "other expenditure including capital expenditure and transfer to reserves".
We asked for some examples of "capital expenditure" and we were told it included garden machinery at Prince Charles' estates at Highgrove in Gloucestershire or Birkhall in Scotland.
They could put these costs somewhere else - but they chose not to.
"That's just the way we've always done it," came the explanation from a royal aide when I asked why they aren't more transparent with the costs of William and Kate, and Harry and Meghan.
The £5.1 million is a big rise on the £3.5 million in the same box in 2017-18.
It also means we can't work out what private contribution the Prince made from his private income to pay for Harry and Meghan's wedding.
The wedding wasn't a state occasion so Prince Charles could have paid for it from the £3.2 million tally of "non-official expenditure".
But we weren't told when we asked.
The Duchy of Cornwall was first created in 1337 by Edward III for his son and heir to provide an independent source of finance from the Sovereign.
It's neither a new source of income nor one over which the taxpayer has any valid claim for more transparency.
So, unless they decide otherwise, the exact costs of William and Harry, their wives and children will remain hidden amongst the machinery in the garden shed.