I tried to find in the documentation what is the calculation used to ad “standard error” error bars on a chart, but could not find any explanation.

I tried to create a chart with two versions:

- the automatic standard error error bars from the “Insert > error bars” menu
- the same but with my own calculated values for standard error of the mean, using the formula
*Std err = Std dev of the sample / square root of the sample size*.

Here is the spreadsheet.

Could anyone explain to me why the resulting error bars are different from one to another?

Cheers