ComputeHash调用莫名其妙地不同

Question

为什么以下两种调用ComputeHash的方法导致不同的结果长度？似乎代码应该产生相同的结果。

byte[] KeyBytes = Convert.FromBase64String("KgMLuq+k1oCUv5bzTlKAJf/mGo0T07jTogbi6apcqLa114CCPH3rlK4c0RktY30xLEQ49MZ+C2bMyFOVQO4PyA==");
byte[] MessageBytes = System.Text.Encoding.UTF8.GetBytes("ae06fcd3-6447-4356-afaa-813aa4f2ba41;70aa7c25-c74f-48be-8ca8-cbf73627c05f1418068667");

// works
byte[] HashBytes1 = new System.Security.Cryptography.HMACSHA256(KeyBytes).ComputeHash(MessageBytes); // correct - 32 bytes

// doesn't work
System.Security.Cryptography.HMAC hmac2 = System.Security.Cryptography.HMACSHA256.Create();
hmac2.Key = KeyBytes;
byte[] HashBytes2 = hmac2.ComputeHash(MessageBytes); // wrong - only 20 bytes