Why are different abbreviated months given for the same culture on different machines?

I am getting a different month abbreviation for the same culture as a work colleague.

var x = new DateTime(2023, 9, 27);
Console.WriteLine(x.ToString("MMM", new CultureInfo("en-GB")));

When targeting .Net 6, on Windows 10 I get Sep, but on Windows 11 I get Sept.

The default DateTimeFormatInfo.AbbreviatedMonthNames are being used in each instance. Why would this happen?

It seems that the OS is the reason for this behaviour. The Microsoft CultureInfo docs (as mentioned in another Stack Overflow answer) state the following:

> The culture names and identifiers represent only a subset of cultures
> that can be found on a particular computer. Windows versions or
> service packs can change the available cultures. Applications can add
> custom cultures using the CultureAndRegionInfoBuilder class. Users can
> add their own custom cultures using the Microsoft Locale Builder tool.
> Microsoft Locale Builder is written in managed code using the
> CultureAndRegionInfoBuilder class.

It's a bit unclear if this just means that the set of available cultures can be added to and subtracted from or if the content of the cultures can be edited. However, the different abbreviated month names seen by my colleague and I imply that the contents can be modified by an OS.

This is important to note as many devs might assume that a given culture would be the same on any OS.