My comment was specifically in response to MikeWh's comment...
...which specifically says the word "overcharged". If there's no overcharging, how are you overcharged?
Aha I get you. I think he's referring to journeys to or from PAYG stations other that Gatwick, when passing through the p13&14 gateline - when using said gateline it could split up a single through journey into two journeys and that could be detrimental.
An example - if you had a zones 1&2 Travelcard-on-Oyster and are travelling from Rayners Lane (zone 5) to East Croydon (also zone 5), you should only pay £3.00 peak / £2.70 off-peak (the multimodal fare for zones 3-5) once.
However if you passed through the p13 & 14 gateline at Victoria that would break the through journey up (as there's no OSI configured) and you'd pay the extra for the zones 3-5 travel twice over (specifically £2.40/£1.50 for Rayners Lane/zone 5 to zone 3 at the TfL rate, and £3.00/£2.70 for zone 3 to East Croydon/zone 5 at the NR rate).
Likewise if you were just doing a straightforward PAYG journey, it would be charged as two journeys (split at Victoria) rather than a through journey. (I'd hope this doesn't happen when using contactless as the fares are calculated in the back office.)
It would only happen if your train from Victoria to East Croydon was using platform 13 or 14 - most of the time this doesn't happen.