I doubt they would be doing that.
The calculation of the number of shares on the buy screen would be at the 15 minute delayed price. The actual goes through at the live. If you check the live price, that is what it should go through at (can differ by a few cents depending on the liquidity of the share and how fast it is trading). I just tested R89.78 (live price) of EC10, with 15minute delayed at R89.70. Actual went through at R89.808 (yes 3 decimals on the cents). The fraction of a cent can make a difference on the number of shares depending on the price of the share (cent fractions on cheaper shares, more impact; expensive shares, less impact) and Rand value you are are purchasing.