Quick Question - rules for heavy bows

[Donny Brook]

Originally Posted by Varyon

...
As for exactly what effect using an ST 14 bow has over using an ST 12 one, a longbow for example deals thr+2 imp and has Range x15/x20. So an ST 14 bow does 1d+2 imp and has Range 210/280, while an ST 12 one only deals thr+1 imp and has Range 180/240.

But where do those figures come from?