Why is the list of derived classes incomplete?

I'm looking at a text file here and .Net tells me the Encoding class is SBCSCodePageEncoding, yet this does not appear in the above documentation as one of the derived encoding classes, why is this?

---
#### Document Details

⚠ *Do not edit this section. It is required for docs.microsoft.com ➟ GitHub issue linking.*

* ID: e108a631-9747-7d6e-dc36-280b0591cd68
* Version Independent ID: f630aa07-55fd-5352-f0c3-bd2e263cd70b
* Content: [Encoding Class (System.Text)](https://docs.microsoft.com/en-us/dotnet/api/system.text.encoding?view=netframework-4.7.2)
* Content Source: [xml/System.Text/Encoding.xml](https://github.com/dotnet/dotnet-api-docs/blob/master/xml/System.Text/Encoding.xml)
* Product: **dotnet-api**
* GitHub Login: @rpetrusha
* Microsoft Alias: **ronpet**