Dispersion and reload time. You may have an extra point of range with the self bow, but you will miss more.
Chances are that early game will take more than a handful of shots to kill a Z and the 10 point difference in reload can add up to a few more squares of breathing room as you retreat with the short bow.
Is Z move around 120? If so, 3 moves (at 100 speed yourself) will get you a free shot with the short, but self would take 4 squares of move (or 3.5, but you can’t move .5 of a square). You can imagine the differences in the bows as you plot that graph out in your head where the short bow gives you more shots over more time as you use it.
You will fire the short bow enough before your next upgrade that there will be a difference.
Edit: Actually, that brings up a thought. Is the moves per attack applied when you fire a bow also? If so the self bow is 135 and the short is 94 giving even more reason to use it over the self. These little easily calculated situations help me decide the tougher ones like which rifle to use for single-targets in a field or what smg to use around a corner in a city.