The bottom 2 curves represent even more "units" (parts) that they tested. Basically, the curves show that the various parameters (hFE, hIE, hRE, hOE) are highly variable from part to part. You cannot just "use" one of the curves -- you have to design for worst-case values across all parts.
On page 2 of the datasheet when they say that hFE ranges from 100 to 300 at Ic=150mA, Vce=1V, they really mean it. It can vary anywhere in that range under those conditions, and as you see further up the table as you change Ic then hFE changes too.
The Rugged Motor Driver: two H-bridges, more power than an L298, fully protected