English vs Italian bottom bracket - chainline considerations
As you have mentioned, the point of having a variety of spindle lengths is to allow selection of a BB which creates the proper chain line, while ensuring proper clearance from your frame. (Typically 51 mm from BB center to the center of the chain ring set.

The assumption that using the Italian BB mandates a wider BB spindle is not correct.  You should have no issue using a 70 mm x 113 mm Italian BB.

That said, you are unlikely to notice any difference in shifting performance from a 1 mm difference in chain line, unless your original BB was already significantly offset (incorrect).  It's just not enough difference to matter.  So if you already have the 70 x 115, use it.  You'll be fine.  If you still need to buy the BB, order a 70 x 113.  

To directly answer what I think is your main question, There is no reason for the spindle length to change, except the convenience of using parts which are already on hand.

I hope that helps.