This question already has an answer here:
In this case BHX-FRA return (Lufthansa) is about £140; FRA-WDH return (Air Namibia) is about £500 - and BHX-FRA-WDH return, on exactly the same flights is about £900.
I understand that this is common, and that a single BHX-FRA-WDH ticket is actually worth more to the holder, because:
if your Lufthansa flight to FRA is late, a single ticket places the airlines under an obligation to look after you, but if you bought them separately, the staff at the Air Namibia counter may well be very sorry but they can't help you...
if you buy separate tickets, you will have to collect your luggage at FRA and check it back into the system
So I can also understand that there is added value in the single ticket.
Questions that have not been previously clearly answered (or posed)
Many questions around this topic have previously been asked and answered, but the first two below have only been answered in passing, while I have not seen any clear answers to the second two.
Who gets the extra £260?
Do Lufthansa and Air Namibia share it out equally?
How does an airline even know that the tickets being purchased are just one leg of a longer journey?
Is there some way of beating the system, by having the separate tickets reunited after purchase into a single ticket, so that for example luggage can be automatically routed to its final destination when it's checked in?