Illustrator video rulers measurement explained

When creating a lower third in Illustrator, I select New > Film & Video > HDV/HDTV 1080 (1920 x 1080 px). This provides a blank artboard with the green video rulers and center mark, cross hairs and video safe area boxes from the Video and Film profile (as expected). 

The right side vertical ruler shows a maximum marking of 1080 (as expected), but the top horizontal ruler shows a maximum marking of 1440 (not expected; this was expected to be 1920). I verified that I had selected the correct template by creating a normal rectangle to dimensions 1920 x 1080 and placing it inside the video frame, which it filled entirely (as expected).

My question is: what is the horizontal video ruler measuring that a 1920x1080 video frame measures 1440 px wide? Even more confusing, when I create a blank document using New > Film & Video > HDV 720 (1280 x 720 px) and New > Film & Video > 4K UHD (3840 x 2160 px), the horizontal and vertical video rulers show the measurements expected. It's just the horizontal video ruler for 1920x1080 that shows a different horizontal measurement of 1440. Is this just a bug?