I didn’t mention the refund amount in my email, just the route and included my journey history.
I just checked the Hertford East to Fairlop fare which £3.55 – I assume the default route is via Stratford (z3) which suggests £4.75 from Hertford N is right. The Stratford route wasn’t available yesterday due to engineering work.
I think the Turnpike Lane fare is the anomaly. I note that Hertford N to Arnos Grove is £4.85 via Bowes Park / Bounds Green. Maybe it’s different if you head inwards on the tube vs. Outwards?