So as far as I can tell, ‘standard definition’ was a workaround bridging the gap between analog and digital video signals.
Analog video standardized broadcast television using ‘D1 format‘ : 720×486
6 missing vertical lines in the video ‘height’ allow DV’s dimensions to be divisible by 16, which is the block size of the most used codecs. They shaved off the rows at the top and bottom, the same rows that were often degraded in analog video signals (don’t ask me why, just quoting an article from this great list of video tech resources: Visually represent 16:9 resolution sizes in your browser window)
I think DVD specifications allow for 640×480, it wouldn’t be pillarboxed on a standard full screen display, so long as the DVD is encoded properly, and the playback unit hasn’t had a user controlled setting that would interfere with it.