unsigned int32 is verifier-assignable-to enum A, but unbox.any throws InvalidCastException

Consider the following C# console application.

using System;

class Program
{
    enum A { A42 = 42 }

    static void Main()
    {
        object obj = 42;
        Console.WriteLine(obj.GetType().Name);  /* "Int32" */
        Console.WriteLine(obj is int); /* "True" */
        Console.WriteLine(obj is A); /* "False" */

        A a = (A)obj;
        Console.WriteLine(a); /* "A42" */
    }
}

It compiles to the following:

.class private auto ansi beforefieldinit Program
       extends [mscorlib]System.Object
{
  .class auto ansi sealed nested private A
         extends [mscorlib]System.Enum
  {
    .field public specialname rtspecialname int32 value__
    .field public static literal valuetype Program/A A42 = int32(0x0000002A)
  } // end of class A

  .method private hidebysig static void  Main() cil managed
  {
    .entrypoint
    // Code size       71 (0x47)
    .maxstack  2
    .locals init ([0] object obj,
             [1] valuetype Program/A a)
    IL_0000:  ldc.i4.s   42
    IL_0002:  box        [mscorlib]System.Int32
    IL_0007:  stloc.0
    IL_0008:  ldloc.0
    IL_0009:  callvirt   instance class [mscorlib]System.Type [mscorlib]System.Object::GetType()
    IL_000e:  callvirt   instance string [mscorlib]System.Reflection.MemberInfo::get_Name()
    IL_0013:  call       void [mscorlib]System.Console::WriteLine(string)
    IL_0018:  ldloc.0
    IL_0019:  isinst     [mscorlib]System.Int32
    IL_001e:  ldnull
    IL_001f:  cgt.un
    IL_0021:  call       void [mscorlib]System.Console::WriteLine(bool)
    IL_0026:  ldloc.0
    IL_0027:  isinst     Program/A
    IL_002c:  ldnull
    IL_002d:  cgt.un
    IL_002f:  call       void [mscorlib]System.Console::WriteLine(bool)
    IL_0034:  ldloc.0
    IL_0035:  unbox.any  Program/A
    IL_003a:  stloc.1
    IL_003b:  ldloc.1
    IL_003c:  box        Program/A
    IL_0041:  call       void [mscorlib]System.Console::WriteLine(object)
    IL_0046:  ret
  } // end of method Program::Main

  .method public hidebysig specialname rtspecialname
          instance void  .ctor() cil managed
  {
    // Code size       7 (0x7)
    .maxstack  8
    IL_0000:  ldarg.0
    IL_0001:  call       instance void [mscorlib]System.Object::.ctor()
    IL_0006:  ret
  } // end of method Program::.ctor

} // end of class Program

Although obj refers to a boxed Int32 rather than a boxed A, the unboxing conversion (A)obj succeeds at run time. This behavior is specified in Standard ECMA-335 "Common Language Infrastructure (CLI)", 6th Edition:

1. §I.8.7 ("Assignment compatibility"): "The underlying type of a type T is the following: 1. If T is an enumeration type, then its underlying type is the underlying type declared in the enumeration's definition. 2. Otherwise, the underlying type is itself." Thus, the underlying type of both A and int32 is int32.
2. §I.8.7: "The reduced type of a type T is the following: 1. If the underlying type of T is: […] c. int32, or unsigned int32, then its reduced type is int32." Thus, the reduced type of both A and int32 is int32.
3. §I.8.7: "The verification type (§III.1.8.1.2.1) of a type T is the following: 1. If the reduced type of T is: […] c. int32 then its verification type is int32." Thus, the verification type of both A and int32 is int32.
4. §III.1.8.1.2.3 ("Verification type compatibility"): "A type Q isverifier-assignable-to R (sometimes written R := Q) if and only if T is the verification type of Q, and U is the verification type of R, and at least one of the following holds: 1. T is identical to U." Thus, A is verifier-assignable to int32 and vice versa.
5. §III.4.33 ("unbox.any – convert boxed value type to value"): "System.InvalidCastException is thrown ifobj is not a boxed value type, typeTok is a Nullable<T> andobj is not a boxed T, or if the type of the value contained in obj is not verifier-assignable-to (§III.1.8.1.2.3) typeTok." Thus, InvalidCastException is not thrown.

However, if I change the initialization to "object obj = 42U;", then Microsoft .NET Framework 4.5.1 actually throws InvalidCastException. Why does that happen? If I understand correctly, the reduced type and verification type are int32 in this case too, so the unbox.any instruction should not throw InvalidCastException.