order price filled early

Hi, Earlier today I traded one lot of ICICI Bank futures. I put a bracket sell order at ₹339 with 337 target and stop loss at 340, however, the price got filled at 339.20 and price traced back and triggers the stop and I’m using SAMCO stock note trading app. Does anyone know why? Thanks.

Sell order executes at the higher of current market price or order limit price.
Then it’s upto the market to go up or down…