Correct.
So with 1080i, you really are only truely seeing 540 lines.
With 720p, you are seeing 720 lines.
This means that 720p is higher resolution, as it shows more info.
Also note that while all HDTV's show 1080i and 480p, some can not handle or support 720p (or some will downsample it to 540p).
The thing is, people that own "true" 720p units can watch "true" 1080i any time.
The people with "true" 1080i units can not watch "true" 720p (540p limit).
Those with 720p units can see for themselves what is better. 1080i can not.
This is not to say the 1080i is bad. It IS great!! But 720p is a tad better.
There are plenty of websites with 720p/1080i info, with most already very outdated.
Here is one that is fairly current, and might help explain why 720p is better:
http://www.videoessentials.com/D_TheaterQA.php