I suspect what you are seeing might be the effect of a promotion they are running (were running? It was supposed to end on Nov 30th) where you get a free upgrade into Club Europe on the connecting leg if you book into the W/E class of WTP, and under various other conditions.
https://www.britishairways.com/en-gb...us-club-europe
T&Cs from same:
- Customers who book selected qualifying World Traveller Plus British Airways (BA) flights in World Traveller Plus with a connection onto a BA short haul flight will be eligible to be booked into Club Europe. Customers booked in W class on long haul will be booked into the lowest available Club Europe class up to and including D class, customers booked in E class on long haul will be booked into the lowest available class in Club Europe up to and including R class. Where the relevant classes are not available customers will be booked into Y class in Euro Traveller.
- Certain route exclusions may apply.
- Travel in Club Europe is subject to availability at the time of booking.
- Customers who change their travel arrangements after the date of booking will only be seated in Club Europe subject to the new arrangements meeting the eligibility criteria.
- There is no limit to the number of times a customer can take advantage of the offer.
- Avios will be awarded for the cabin flown in.
- Booking dates: 15 October 2019 – 30 November 2019.
There was some discussion here, but my Google-Fu is failing me for the moment. It may have been silently extended, or still be active in the booking engine.