What is the difference between SftpClient.UploadFile and SftpClient.WriteAllBytes?

I am observing some strange behaviour when I use SSH.NET to transfer files with SFTP. I am using SFTP to transfer XML files to another service (which I don't control) for processing. If I use SftpClient.WriteAllBytes the service complains the file is not valid XML. If I first write to a temporary file and then use SftpClient.UploadFile the transfer is successful.

What's happening?

Using .WriteAllBytes:

public void Send(string remoteFilePath, byte[] contents)
{
    using(var client = new SftpClient(new ConnectionInfo(/* username password etc.*/)))
    {
        client.Connect();
        client.WriteAllBytes(remoteFilePath, contents);
    }
}

Using .UploadFile:

public void Send(string remoteFilePath, byte[] contents)
{
    var tempFileName = Path.GetTempFileName();
    File.WriteAllBytes(tempFileName, contents);
    using(var fs = new FileStream(tempFile, FileMode.Open))
    using(var client = new SftpClient(new ConnectionInfo(/* username password etc.*/)))
    {
        client.Connect();
        client.UploadFile(fs, targetPath);
    }
}

Edit: Will in the comments asked how I turn the XML into a byte-array. I didn't think this was relevant, but then again I'm the one asking the question... :P

// somewhere else:
// XDocument xdoc = CreateXDoc();

using(var st = new MemoryStream())
{
    using(var xw = XmlWriter.Create(st, new XmlWriterSettings { Encoding = Encoding.UTF8, Indent = true }))
    {
        xdoc.WriteTo(xw);
    }
    return st.ToArray();
}