It doesn't make a mono signal into stereo. It simply puts it down the center in the track.
Mono can be panned to the left or right in a track.... since you're generally working with a STEREO output buss.
True stereo would in fact have 2 different wave files in the track
A picture is worth one thousand words so.....
notice the dual wave forms in tracks 5 thru 8 while the others are single waves in a track ...best seen in tracks 13 thru 15.
The dual waves are true stereo tracks with unique audio information in each channel, whereas the mono tracks, aside from the BB/RB generated ones, are all recorded by the left input channel of my interface. However, they are mono and therefore are straight up the center and totally pannable in my stereo output buss.
Hope that clarifies it a bit.