No, they are not different plants so I have no idea why the two different family names. They are both considered what is commonly known as butterfly bushes.
From ITIS Buddleja davidii is listed in the Scrophulariaceae family:
http://www.itis.gov/servlet/Si...
and when I entered Buddleia I got a 'No Records Found report:
http://www.itis.gov/servlet/Si...
Here is the whole ITIS list when I entered Buddleja:
http://www.itis.gov/servlet/Si...
well shoot, neither one of those last 2 'SingleRpt' links work, just the first one worked. Looks like you have to enter the genus yourself.
