I would imagine that just indicates that the extension fare from Zone 5 to Potters Bar, which is unpublished, is more than the point-to-point fare for New Southgate to Potters Bar.
The rip-off unpublished extension fares strike again... under the Consumer Rights Act, if a price is not agreed before the service is provided, only a "reasonable" price may be charged.
I fail to see how TfL can claim that the inflated extension fares are "reasonable", particularly given that there's nothing that TfL publishes which warns that extension fares may be more expensive than point to point fares.
Of course, whether TfL will willingly refund you the difference is a different question to whether they're obliged to do so. If they refuse, you'll have to consider whether it's worth your time and effort to pursue the matter further.
But I'm sure I've actually purchased tickets in the past with Boundary Zones from ticket offices? And even TVMs?
The 'paper' Boundary Zone fares (in some cases they may also be able to be fulfilled as e-tickets or on a smartcard) are completely separate to the Oyster/contactless PAYG extension fares.