This is caused by how the production sorter works. It takes into account the total amount of produced cargoes.
(1,000 litres of oil = 1 item, 1 passenger = 1 item, 1 tonne of coal = 1 item)
For example:
Tudningstone Oil Rig produces 336,000 litres of oil and 12 passengers, in total 348 items
Munford Oil Rig produces 342,000 litres of oil and 6 passengers, in total 348 items
Canley Oil Rig produces 315,000 litres of oil and 30 passengers, in total 345 items

Thanks for the reply.
I can understand that if I choose "All cargo types" the sorter combines the two cargoes, but shouldn't it just look at whatever is chosen when only one of them is selected?
Thanks!