Wasn't allowed to check in because I was 4min late

On the 11/02/2016 I had a flight booked from Lanseria to King Shaka international for 18:20. I got to the check in counter at 17:48 and the lady had began to check me in. She had requested my ID and weighed my hand luggage which was the only piece of luggage that I had. Then she told me that I can not board the flight because I'm late. I could not understand why she began to check my in if I was late. I pleaded with the guest services to allow me on to the flight because I was only 4 minutes late and had no luggage to check in but they flat out refused. I am extremely disappointed with Mango airlines and since I'm a frequent traveler I have used their services multiple times and that's the thanks you get for supporting mango. While I was pleading with the guest services they were still doing final boarding calls for the flight so I can't understand why I wasn't let on to the flight. The only other reason was that Mango as usual overbooked their flights therefore the lady began my check in and then suddenly refused to let me on to the flight. This type of service is despicable and a huge cost and time inconvenience.