Hi Adam,
Sorry for the delay in responding. You’ve answered one of the points in that yes, you aren’t dealing with OSIs.
The best way to explain the surcharge is to give examples. Purley (zone 6) to London Bridge (1) costs £4.30 off-peak. That is a National Rail only journey. If you extend it to be Purley (6) to Oxford Circus (1) then the fare becomes £5.80. The extra £1.50 is the surcharge for mixing National Rail and Underground on a journey involving zone 1. Looking alternatively at Upminster (6) to Fenchurch Street (1) the off-peak fare is £3.30 because even though it is National Rail, that line charges the same fares as the Underground. The fare thus does not increase if you extend it further into zone 1 on the Underground. Note that any fare (with a surcharge or not) is limited by the daily cap for the zones used, so if you make lots of journeys you’ll end up paying the same amount.
Sadly, Gatwick is a complicated place. The fare between there and East Croydon (5) is historically low and this caused issues when people used contactless to commute for a day. Rather than paying the full Gatwick to zone 1 single fares each way the system split it up as a zone 1-4 cap and two singles Gatwick to zone 5. The train company weren’t happy with this and so the contactless extension fares were fiddled so that the ‘right’ fares were charged. With Oyster it’s even worse because if you start at Gatwick the system will continue charging until the Gatwick cap is reached. The recently added overnight recalculation should queue a refund which would be picked up the next day if all the rest of the travel is in zones 1-2. The net result is that if you start at Gatwick and zap around zones 1-2 without returning beyond zone 2 then you should be charged £8.70 + £7.40 = £16.10. This is always assuming off-peak fares.
If you break the journey at East Croydon then it should become £3.40 (Gat – EC) plus £2.70 (zone 3-5 extn) plus £7.40 (zone 1-2 cap), totalling £13.50. Contactless should charge that straight away but Oyster will again overcharge and refund the next day. Again off-peak fares are assumed.
I hope that helps explain things a bit.