There are some destinations off the rail network that you can book through to on a rail ticket (other than the 'plusbus' scheme that is) - for example, i think you can book train tickets through to minehead by changing to bus at taunton.
Fares don't normally change if there's rail replacement bus / coach in operation.
As for this specific, I think NRE has got its electronic underwear in a tangle.
I've just tried for the date you say and current time, so it's giving departures after 2215. The journey details say its an Abellio Greater Anglia service, calling at Bishops Stortford and Tottenham Hale, taking 47 minutes to Liverpool Street. Which seems right.
It's offering me a 12 pound single with the following blurb -
Bus Day Single
Anytime : fully flexible tickets with no time restrictions on when you can travel. Perfect for people who need flexibility
ROUTE OF TICKET A6 VIA PADDINGTN - By National Express A6 coach via Paddington
Which does not make a lot of sense.
I've tried some on a different day, mid morning, and it's suggesting a 19 pound single with the text I'd expect for an 'any time' single.
[ETA - largely duplicates what other people posted while I was researching, but such is life...]