A stop order is an order to sell/buy at the market once a certain price is reached. So for instance in this situation once the BID reaches .84909 the broker would fill your order 'at the market'. That price high on the chart of .84891 (indicative as already mentioned) is the mid price at that moment. What would the bid have been then? Spreads are upwards from around 3 pips for the Aussie and your platform is showing five decimal places (the fourth decimal is the whole pip value), so I would say the bid would have touched that level.
As mentioned stops involve market orders, so theoretically it is quite possible to have positive slippage. There could be a handful of trades at that top however once your stop was triggered market price could have started slipping back before execution. That said, in your case it doesn't look like this happened, its just normal execution with the effect of the spread.
What can be done to avoid this? Well your stop levels need to take the spread into account. Also that level could be said to be a popular level for stops - you'll see in the PA that traders probably put even more stops at that level 'above resistance' which is why price popped up there again in the afternoon. Perhaps avoid those obvious spots. You could trade futures (if you've got more funds) where the market is open and you won't have a broker taking the other side of your trade, and knowing where your stops are. For short term trading though that is of limited value - at the end of the day any short term trader with experience would know that level was a good one to target for order flow, and at the end of the day thats what makes money for bank traders, brokers and successful short term traders. Its all part of the game!