Fares SE terminals to GN/TL stations on fare finder

One bonus of the Thameslink route connection to the Great Northern line is being able to change at London Bridge for Waterloo (E), Charing X or Cannon Street so being able to get to the West End or City without having to use the tube. Even better, it’s also cheaper because the ‘London terminals’ rate is charged. All three termini have fares on Fare Finder to the original Thameslink stations (Kentish town to St. Albans) but only Waterloo (E) has fares to Great Northern stations (Finsbury Park to WGC / Hertford N.). Any idea why? I have used contactless from Charing X to Hertford N via London Bridge and been charged the correct London terminals fare.