A month ago I wrote that HMRC's claimed 'compliance yield' was, to be polite, pretty much made up.
The National Audit Office put it more kindly in their report on HMRC's accounts:
We also consider that HMRC should have been more transparent in reporting its compliance yield and describing what it included. In past years it has not made it clear enough that only some of the yields are in-year revenue benefits, or that there are uncertainties in the data. Unlike other areas of its reporting, such as on the tax gap, HMRC has not produced sufficient detail describing its methodology and approach.
I think that comes to pretty much the same thing: they pretty much made it up.
And even so, they got it wrong.