like I’ve said, the TBB rounds the numbers, based on your original resolution. As far as I can tell, 1000x600 happens for things arround 720p (and everything in between like 1366x768) and 1000x800 arround 1080p, though I’m not sure, this is just based on what experience I’ve had with my computers. As long as it is not matching up with your real resolution though, it works properly.
Also, ip-check.info really isn’t a good test in regards to the TBB, as it’s incapable of understanding how the TBB provides anonymity (by giving everyone the same fingerprint with only minor variations).
Have a nice day,