There are two things I don't understand here: Firstly, why in the standard deviation expression we use the Q operator instead of the Q value itself? Writting Q seemed more natural and then in the next expression I would just stick the corresponding operator. Which would lead to the same result or is it just two errors cancelling each other? Secondly, why in the last expression we are not using an operator of the quantity that was in the previous expression under the "average" brackets? Is it an operator of itself like coordinate x is for example?