Unfairly treatment

My name is Olerato Sebego and I have a story to tell.

A few months back I app**** for a peak relief post at Bloemfontein FNB in Charlotte Maxeke street and got called for an interview, I went for it however was not successful because I wasn't well prepared. I went back, did some research on FNB and I took my CV back to them again with the hope that someday something will come up again. On the 15th two weeks ago I got an sms stating that I have to avail my self for an interview on the 21st at 11:00 and I started preparing myself on some of the questions I still remembered and the research I did. I showed up and my interview went well, I received a call from a lady called Refilwe same day later to tell me that my interview was a success but they just have to check my ******** records and all. I was happy that everything went well , and she gave me another call and told me that I'll be starting on the 1st of September. All of a sudden on Friday after I got the date the branch manager Mr Koos called me and told me that the job is not mine anymore and HR chose someone else. I didn't understand, I mean why would a company like FNB make a mistake "of hiring the wrong person"? or tell someone that they got a job and next thing snatch that opportunity from them again? I am not being bitter about this or anything however I just want to understand what went wrong? What did I do wrong? Because I honestly do not think what happened was fair. I was advised that I can go to the CCMA with this issue however I just feel that it's better if I first understand what really went wrong in this ordeal.

Regards. Olerato Sebego