I'd like to say that BA wants to ensure a good quality product, given staffing levels, food and beverage stocks and to minimise your expenditure on therapists. But I suspect that BA's IT may have a few small imperfections, given (e.g.) I cancelled my flight 12 hours ago and the availability is still not back in the market. The most bizarre example is if you look at Avios in CE availability on the day of travel, you can sometimes find a CE RFS available and J0! There are some routes when they don't fill the aircraft (e.g. Skiathos) and some combinations of CE versus ET they won't handle due to crew workload.