Why the document mode is only changing on setting compatibilty mode for IE in IIS ?

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ASP.NET

IIS

C#3.5

Why the document mode is only changing instead of browsing mode on enabling the compatibility mode of IE 11 browser in IIS ?

I want to enable the compatibility mode of IE 11 browser in IIS as per below steps:
Open Internet Information Services (IIS) Manager and browse to the site to be configured.
In the HTTP Headers tab, click Add in the Custom HTTP headers box.
In the Add/Edit Custom HTTP Header dialog, enter X-UA-Compatible in the Name field and IE=11 in the Value field in IIS.