Why do cartridge manufacturers give a range for VTF?

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.

I think it should be a specific number....there is only one spot that aligns the coils et al as it was designed. 

But after you set static VTF, in actual use the cantilever is moving constantly. How much is to some degree affected by tonearm inertia. So there is no one exact correct value for VTF. Also no two samples of any cartridge are exactly alike in terms of compliance. Another reason to provide a range of values.

Lyra Delos also has a VTF range; it's 1.7g - 1.8g with 1.75g being recommended. 
All that means is that they want you to start at 1.75g and move on from there. This is no different than any other manufacturer