c# Convert.To/FromBase64String confusion

Assuming I have this Method.

private static void Example(string data)
    {
        Console.WriteLine("Initial : {0}", data);

        data = data.PadRight(data.Length + 1, '0'); 
        Console.WriteLine("Step 1 : {0}", data);

        data = data.PadRight(data.Length + 4 - data.Length % 4, '=');
        Console.WriteLine("Step 2 : {0}", data);

        byte[] byteArray = Convert.FromBase64String(data);
        string newData = Convert.ToBase64String(byteArray);
            Console.WriteLine("Step 3 : {0}", newData);
    }

I expect the output given the input string "1" to be as follows

Initial : 1
Step 1 : 10
Step 2 : 10==
Step 3 : 10==

Instead the output is this.

Initial : 1
Step 1 : 10
Step 2 : 10==
Step 3 : 1w==

And I have no idea why. I would expect the output to be the same as the input but it isn't. I have tried replacing

data = data.PadRight(data.Length + 1, '0'); 

with

data = data + "0";

It appears with longer input strings too, for example strings with a length of 5 or 9. It works fine if I add "=" but then I exceed my padding limit with Convert.FromBase64String()

So my question is really what is going on and how can I get my expected output,? What am I doing wrong?

Edit: For those confused as to why I'm using bas64 it is related to this PHP decrypting data with RSA Private Key