Why do cartridge manufacturers give a range for VTF?

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 

I agree with stringreen's argument while the reason is my Allaerts
MC 2: 1,8 g ( maximal tolerance 0,05g). However chakster's
''damping argument'' should also be of some importance because
the damper is behind the coils and, depending from its ''hardness''
should have some influence on the VTF. In my opinion Allaerts
demonstrate that getting optimal position of the coils is not an 
easy proposition. J. Carr wrote about this problem as well his
solution to the problems by his new cartridges. His intention is
also to get those ''tolerances'' as small as possible.