If you read through the HOC 68/97, para's 7 to 10 inclusive they explain the reasons why they feel they should not be affected by the legislation.
In effect what they've been saying for 18 years, and recently (a couple of days) confirmed is that the legislation should in fact have excluded them directly and they omitted to do so.
The Home Office advice has been consistent over the last 18 years and it's only now the GTA find a problem. Could it be that it's because people are buying from outside their influence ?
As an example you can have a semi/full auto paintball marker that shoots at around 10FPE (possibly more) because they are specifically excluded. Now that seems a nonsense if you cant have a semi sub 12FPE air rifle.