Shamrock - your data tells me that BOH focuses on larger aircraft (eg 189-seat 737) taking people both arriving on holiday (particularly UK holidaymakers wanting Bournemouth's beach when Covid made foreign travel more difficult) and also going on holiday while Southampton which lacks a beach of its own focuses on smaller aircraft which are probably more business-centric and thus less in demand in August
Data for September 2021 would be useful to see what happens when the UK is not in its annual period of beach mode