Trades Initiating before Buy Stop MT4

Just a quick question, I’m new to this so apologies in advance if this is a silly question… I’ve just started trading and ive noticed a couple of times now that when i set a buy stop the trade is inititated before it reaches that point, this morning for example i set the buy stop at 0.89311 and the trade has initiated at 0.89299 am i using a buy stop wrong or are there are other factors im missing? Using Meta trader 4 and IG as my broker if that makes a difference, thanks in advance.