Robin 'throat' singing?

Hi, I'm wondering if anyone can help me. I've got a robin coming to my London garden who is, of course, very friendly. I named him Gary. Now, he often sits out there singing away, but with his mouth closed, like 'throat' singing (see video link attached). Why is he doing this? Is he practising? On the video he is quite loud, but that's because I was close, his singing when he does this is actually very quiet and sounds like he is far away. I've never seen Gary singing fully, though I have perhaps heard him, but that could have been any number of robins in this area. Any ideas gratefully received! Cheers Jane