MetaTrader charts are created using the bid price. Closing of buy orders is triggered by the bid price so you will see the chart line hitting the SL (Stop Loss). 

However, the closing of sell orders is triggered by the ask price, which is not visible by default. That’s why 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, opening properties and enabling it.