Why is Matlab's control chart using standard error to calculate control limits?

I am trying to generate a 'c' control chart with the following code:

[stats, plotdata] = controlchart(cData, 'chart', 'c');

Which generates the figure shown below and returns the associated statistics. The mean is calculated correctly as 1,107, but the LCL should be 0 and the UCL should be 3,363. The standard deviation of my data set is actually 751 and then the CLs should be calculated from this, however, Matlab looks to be using the standard error, which is calculated as 33 (thus the LCL is 1007 and the UCL is 1207 (mean +/- 3*se) as shown in the figure). So my question is how can I get the controlchart function to correctly calculate the standard deviation? Note that I tried doing this as well, to no avail (in fact it didn't change anything):

[stats, plotdata] = controlchart(complaintsPerMonth, 'chart', 'c', 'sigma', 751);

Thanks in advance for any help!

[the cyclist]

I'm no expert, but the Wikipedia page on control charts states in more than one place that the chart uses standard errors.

[KAE]

No expert either, but this does not look like a normally distributed process with a mean value that stays pretty constant over time. Also it seems like the input argument 'sigma' can be your choice of method for estimating the upper and lower bounds. Look at the controlchart.m code,

edit controlchart.m

and you will see the choices.