What determines the resolution GiP selects for download?

[Nick Payne]

I've been downloading some of the coverage of the world snooker 
championships. All the downloads use the command line

get_iplayer --pid="%1" --modes=hlsbest

but I find that the resolution of the downloaded mp4 files varies quite 
a lot. Most are 1280x720, but some are 960x540, some are 832x468, and a 
couple are only 704x396.

I retried one of the downloads that was only 704x396, using the same 
command line with the addition of --force, and the second time it 
downloaded at 832x468, so I'm wondering how GiP determines what 
resolution to download?

I'm running GiP v2.94 on Windows.

Nick