Actually, his numbers have gone up. But any poll that asks “adult Americans” their preference is a bad poll. You know it.
Romney's poll numbers have hoovered around 22% + or - 2 points or so over the last month.
You can ignore one poll as a possible outlier, but when every single poll for a solid month from different pollsters using different methods shows the race at a particular point... you can be pretty darn sure that's where the race is at.
The poll asked Republican registered voters about their choice for the Republican nominee.