Woah!!!!
That’s insane. The fare of £6.10 is for Hertford North to Turnpike Lane changing between Bowes Park and Bounds Green. I think the Upney fare might be a mistake, but go for it and see what they say. The other alternative routes for Hertford North to Upney are charged at £6.25 (avoiding zone 1) and £6.85 (via zone 1).
The fares system in London is insanely complicated, so it’s no wonder that errors and omissions creep in.