Many flights also seem to fly half empty
Welcome to the airline business.
Average load factor for flights to/from Heathrow last year was 75.6%, which by industry standards is pretty good going.
So for every flight that arrives or departs almost full, there are likely to be others that fly with 50% or fewer of the seats filled.
That's how scheduled operations work, particularly on short- and medium-haul where you can't realistically expect loads on each half of an inbound/outbound rotation to be equal because of time-of-day considerations.