Again the term "their estates" is very loose.
They own them in name only. They can't sell them and aren't directly liable for the costs incurred by them.
But this really is a technicality.
In reality George III gave the estates to the Government to manage, when it was fairly worthless, in return for the civil list.
The monarchy may now get a fixed percentage, but at the time they were handed over, it was a government bail out for the King.