That's confused me even more. So is it last_id or since_id. A one word answer will suffice :)
http://www.daniweb.com/api/forums/3/chat?long_polling=1&last_id=10000
http://www.daniweb.com/api/forums/3/chat?long_polling=1&since_id=10000
http://www.daniweb.com/api/forums/3/chat?long_polling=1&last_id=10974
http://www.daniweb.com/api/forums/3/chat?long_polling=1&since_id=10974
All give the same results - the last param doesn't seem to do anything