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


However, the closing of sell orders is triggered by the Ask price and is not visible by default. Therefore, stop losses of sell orders may occur at prices not visible on the chart. 


The 'Ask Line' can be enabled by right-clicking on the chart and selecting Properties.