Standard Deviation

Guys,
May I ask you to have a look into the formula that is supposed to calculate Standard deviation.

The formula for SD should be:
sqrt(sum(P - SimpleMoving Average)^2) / Count)
where in the spreadsheet is actually:
sqrt(sum(squared P)/count - SimpleMovingAverage^2)

Please see cell I13 --> SQRT((1/10)*SUM(E4:E13)-C15*C15) How come the result is the same as in P23 ???? I am trying to understand "SUM(E4:E13)-C15*C15"

How come the results match???

Thank you for your kind help

Isn't formula for standard deviation:

SQRT((1/(10-1))*SUM(E4:E13)-C15*C15)

or N - 1 in other words