This car park has both council and private (NCP) parking contractors. The signs are so confusing that people pay NCP and park in the Milton Keynes Council's part - and get a Penalty Charge and vice versa.
Here is what NCP have said (July 2015):
NCP are aware of this issue with this car park and ..NCP decided some years ago not to issue PCNs where a valid council ticket is displayed. NCP accepts that local staff are not following this policy and PCN's (Parking Charges) should never have been issued. They accept that unsuspecting motorists should not be penalised for a poor car park design.
Hat tip: Pepipoo link here