seat selection ****??

On BA you can select your seat 24 hours before the flight. If you select your seat before then you pay. I checked about 4 days before to see if I needed to reserve a decent seat. The seat map for my flight showed over 100 economy seats available. The next day I was surprised to see only about 10 available, with one aisle seat left. Rather than waiting until the 24 open time and risk losing the last decent seat, I hurriedly reserved the last aisle seat and paid R926. The next day I checked and more than 100 seats were available again. I contacted BA and suggested that they have system problem. In short , after speaking to several people and queuing on the phone system, I was given the standard answer, "nothing can be done and no refund of the R926". 7 weeks later I again checked the seat plan for my return flight. Again over 100 seats were available. Closer to my flight date I checked again and shockingly it showed very few seats left. Thinking it may be my browser I tried another, I also tried my smart phone. All showed the same few seats left. I quickly reserved a seat and paid R400. As soon as I reserved my seat I checked again and all the >100 available seats were back. Once could be a system, problem, but twice ?? I described this to my friend who said it is a well known practice by BA to get people to pay for seats. I can't believe this. I raised this with many people at BA, and on the advice of Customer Services, the CEO (Mr Cruz). Mr Cruz couldn't be bothered to question this internally and passed it on to ... yep... Customer Services. Again the standard replies, including "what do you want me to do about it", "that's bad, you should write to the CEO"., "its because you booked with the BA Executive Club membership" It's as if BA staff don't work for BA. I attach the seat plan after I booked my seat. It shows many seats available. Has anyone else had the same experience please.