I was ordering stocks of Aston Martin Lagonda (AML) this morning at 10.31am and suddenly the price executed (80.90p) was above the price at that time (80.30p). Im just wondering why I was charged with 80.90 even though it didnt reach that level?
This thread 
Help - Way overcharged for an order 
might be able to answer your question, for a very similar subject. In short, prices can change at a fast pace, and using a Limit Order will lock in your price for no more than you set it. Using a Market Order opens up the risk to paying higher.
I lost over £120 on a sell earlier this morning due to market changing in the blink of an eye. Heart breaking but, education isn’t cheap 
Just checked on Tradingview, pretty unlucky. Worth looking at the detailed 1min charts before buying or selling.
@Cashurkash did you sell at 74?