.... which also has a flat side & rounded side. They're mostly colored with the darker color on the rounded side (top). Darker colors, on a two tone bait, are generally the top color ... representing the darker color of a baitfish's back, to contrast with the lighter color of the belly. A more "natural look", if you will. (even when the colors don't correspond to any known creature :p ) It's their natural camo ... lighter color belly to blend in with the surface reflection (when seen from underneath) and darker color back, to blend in with the darker background of the bottom or dark water depths (as seen from above).
Flat side down also creates a gliding effect, where a round side down usually just falls straight. It's more noticable when using a very light jighead.
I have no experience with the beavertail baits .. but the question seemed somewhat ambivalent towards any particular bait (it does say "soft baits LIKE a yum beavertail"). I just answered it in relation to what I do, and what I'm familiar with.
When any plastic bait I use, has a flat side ... I consider that to be the "bottom" side of it ... be it a worm, grub, or creature bait. Lots of critters have flattened undersides and humped/curved backs (like snakes, nightcrawlers, catfish, etc) .. but, few (if any) have a belly bulge (curved) and a flat back. It just seemed more "natural/normal" to me, therefore, to consider the flat portion of a bait to be the underside.
... cp