MetaTrader charts are created using the "bid" price. Closing of "buy" orders are triggered by the "bid" price. You will see the chart line hitting the "Stop Loss". 

 

However, the closing of "sell" orders are triggered by the "ask" price which is not visible by default. Due to this,  "stop losses" of "sell" orders may occur on prices not seen on the chart.  

 

The "Ask Line" can be enabled by right-clicking on the chart, choosing properties and enabling it.