This often shows up on codeshare flights. BD's fare on its codeshare with AC to LHR ex-YYZ always shows cheaper on Expedia than seats booked under the AC flight number. As often do UA codeshares on ACmetal. For the most part this is because the host carrier tends to sell out it lower-cost inventory more quickly than do the codeshare partners who tend to only use their allocation for connecting flights from/onto their own flights.

In your case, I would not expect LH to do much marketing of their codeshare flight between a non-German city and the UK, so their inventory of cheaper seats was still there, while BD had long ago sold these categories and only had full Y availabilities.