Huh. That's pretty weird. Flights to/from the US/Europe (including the UK) are often a bit weird though, as they try and fill them up, and depending on the timing. Flying to the US I believe at least pre-COVID the optimal booking time was 1 month before going, price-wise, and then there was another, lesser low even closer to flight time. And you sometimes find wacky routes are much cheaper - I know when my wife flew to the US last, flying to Dublin and then from there was cheaper than flying from any UK airport, even though like, no way is that cheaper in terms of fuel and wear and tear and so on. Some destinations on mainland Europe it was the same way - even though virtually all of them involved flying "the wrong way" (at least Dublin was arguably "the right way") and a significantly longer total flight distance.
Air travel - a goddamn eco-unfriendly mystery!