Are you seeing both fares from a website like brfares.com? It doesn't have any logic to work out of the station you put in (St Pancras) is actually a valid London Terminal for the direction you're coming from, it just knows that St Pancras is in the London Terminals group so shows fares valid to London Terminals regardless of validity. As Mcr Warrior said the London Terminals ticket won't be valid to St Pancras, so a journey planner with a bit more logic won't show it.
If your work allows you to claim back a TfL PAYG fare on Oyster or Contactless, the single fares are £5.30 in the peak and £4 off peak for travelling on National Rail only.