Why do cartridge manufacturers give a range for VTF?

Well, normally there is a small range from any manufacturer, but there is also an exact recommended tracking force in any manual i have seen (no matter MM, MI, MC ....). There is also recommendation to burn in a new cartridge with higher tracking force and then lower it down to recommended tracking force. A used cartridge with weak suspension can be used with lower tracking force. So there is a case for almost any situation.  I always use recommended tracking force for 99% of my carts. 

I've noticed that there seems to be a post removed almost every time I post. Do I have a stalker? A'gon is looking into it.

Agree with Chak that most carts have a range but then there will usually also be a recommended number, invariably pretty much in the middle of the range.

I just shoot for that middle ground number every time or slightly higher.

I think too the tone arm the cartridge is bolted onto is a consideration. Perhaps one with less friction in it’s bearings can enable the cartridge to track at lower VTF than one with slightly higher friction. Cartridge manufactures may specify a tracking force range in acknowledgement of that reality.