Every currency pair has a bid/offer spread. For EURUSD, it will have a spread of 1 to 2 pips for most brokers. Thus, if you SELL EURUSD, in order to close your position, you need to BUY. Your stop loss is at 1.3257. That means to close your position, the broker will quote the BUY price of 1.3257. Thus, with a spread of 1-2pips, it is correct that the SELL price of 1.3256 shown on chart will mean a BUY price of 1.3257 or 1.3258. That is why your stop loss is hit