You ask why is nothing easy.
Well by your parents making a false declaration eight years ago when they purchased the property (thereby almost certainly committing a criminal offence) your parents and the seller created the complication.
If everyone was honest and declared the true sum, then I guess, as probably in the UK, the tax authorities would always accept what is declared as being the correct figure. As people are often dishonest the tax man needs to ‘try harder’ to obtain what is due.
In your particular case, public funds were defrauded 8 years ago, as the correct tax was not paid upon the purchase and that dishonest situation has probably continued yearly as subsequent tax returns have been based on a lower figure than they should have been.
Can’t blame ‘the system’ for that.
Bit like a bank robber blaming the bank he robs.