In the case of Pulkovo, their LHR slot goes back to when Aeroflot operated LGW/LHR-St Petersburg - I guess their aim was to have as many flights at LHR as possible, so thats why there was one LHR-LED flight a week. When Pulkovo took over LON-LED from Aeroflot, the once weekly LHR flight came with it, and they havent changed it since