Ben, you're right, it does not make sense. SAA has gone through a tough patch these past few years in terms of management and all parts of operations, and I think that the chop-and-change we see in the fleet selection is a process of that. About four (maybe three?) years ago SAA brought in an American (Coleman Andrews) to run the airline, as it was going under in a serious way. He implemented (for better or worse, and many would say worse) numerous steps to get the airline back into the black, one was to standardise the fleet. It might be unfair, but as an American, he might have had loyalties to Boeing, and as SAA has a long and distinguised relationship with Seattle, that angle was pushed.
Andrews realised that SAA deploying A300 on the CPT-JNB-DUR triangle was causing the company to loose money by using big jets on routes that are not busy all times of the day, and also the airline did not have that many frequencies between those stations on a daily basis. So, 737-800s are picked, and many of them. The strategy: smaller aircraft, more frequencies-this model is apparently the preferred one for yield maximisation purposes.
Crew were dissapointed that the A320 had to go, and the new 737s were giving the company problems, such as poor cargo capacity, poor turn-around times, pilots not happy with flight-deck (this I gather from comments on PPRUNE), the list goes on.
Andrews though made far more enemies than friends at Airways Park in Johannesburg, and suddenly got the boot. (reason: not understanding the complexity of the SA culture, too much of a laissez-faire approach, annoying many airline people by side-stepping them and bringing in US airline consultants, an SAA-internet venture that went seriously belly-up before it hardly started, thus loosing tens of millions of US dollars. This is one side, he also claims that as usual in SA, politics ruled, with gov. looking over his shoulder all the time. However you look at it, he messed up (leaving with a ZAR salary of 200 million).
New CEO, the fleet plan; classic 747s have to go. Boeing and Airbus bid, Boeing with the 777. 777 will not fit SAA fleet, Airbus, keen to get back into the picture, I am sure said to SAA, get rid of pesky 737-800s, and we give you a good deal on 41 odd new aircraft.
As you can see, complex story (far more I'm sure), but I hope it gives you some idea as to why the fleet thing has been a mess.