"what am I missing here?" "Likely voters" are registered voters who say they will vote on election day. "Registered voters" are just those who are registered, but there is no indication of their intention to vote. Truth be told, polls of "likely voters" are considered more accurate.
Good point -- I just like to know who they are polling. It's like when they say "9 out of 10 doctors prefered this med" and you don't know if it's a doctor, a dentist or a vet.