MetaTrader charts are based on the BID price. Closing of BUY orders are triggered by the BID price so you will see the chart line hitting the SL (Stop Loss.)

However, the closing of SELL orders are triggered by the ASK price which this line is not visible by default. 

This is why stop losses of the SELL position may occur on prices not seen on the chart, you can enable the "Ask Line" by right-clicking on the chart, selecting properties and enable.