Why is it possible that my executed price is not within the candle?

One of the possible reason is...
We get a better spread offered by the broker, compared to the default spread displayed on the MT4 chart.

In this case, look closely at the chart spread (default market spread) and price compared with “market watch” (spread we get) . “Market watch” is the one client gets. 

so in such case, chart BID price will be constantly 3 points lower than what we get. So if we are doing a BUY, which will be executed in ASK price, and we want to look at chart history highest ASK price, we get the candle highest price +3 points (due to tigher spread), then +30 points (due to FIX spread)

Below is another example of tighter spread compared to chart.

And to understand between how variable spread and fix spread gets its' BID ASK price. Refer to chart below.
compare "image 1" with "image 2", you will notice that if the variable spread moves, it is possible that the fix spread symbols doesn't move at all for both BID ASK price

image 1

image 2