typeof generic and casted type

In GenericMethod((object) 1), T will be object. typeof reflects that.

But item.GetType(); is a virtual method and will execute at runtime on Int32.

typeof returns the static (compile-time) type of the generic parameter T.

GetType returns the dynamic (run-time) type of the value contained in variable item.

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The difference is easier to see if you make your method non-generic. Let's assume that B is a subtype of A:

public void NonGenericMethod(A item)
{
    var typeOf = typeof(A);
    var getType = item.GetType();
}

In that case, calling NonGenericMethod(new B()) would yield

A
B

Recommended further reading:

• Run-time type vs compile-time type in C#

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Now, you might ask: Why did you use NonGenericMethod(A item) in your example instead of NonGenericMethod(B item)? That's a very good question! Consider the following (non-generic) example code:

public static void NonGenericMethod(A item)
{
    Console.WriteLine("Method A");
    var typeOf = typeof(A);
    var getType = item.GetType();
}
public static void NonGenericMethod(B item)
{
    Console.WriteLine("Method B");
    var typeOf = typeof(B);
    var getType = item.GetType();
}

What do you get when you call NonGenericMethod((A) new B()) (which is analogous to the argument (object) 1 in your example)?

Method A
A
B

Why? Because overload resolution is done at compile-time, not at run-time. At compile-time, the type of the expression (A) new B() is A, just like the compile-time type of (object) 1 is object.

Recommended further reading:

• When is the generic type resolved in c#?

The call to GetType gets resolved at runtime, while typeof is resolved at compile time. That is why it is giving different results. you can check here - When and where to use GetType() or typeof()?