Without more information on how the unit is equipped - mast type,(two or 3 stage), mast lift height, attachments, etc. it is hard to provide a specific reason. But fro starters;
This unit has a basic lifting capacity rating of 8000 lbs. at 24" load center with a given mast type & lift height & forks". When the mast type is changed, say from a two stage to a 3 stage mast, attachments are added, like side shift, fork positioner, fork clamps, etc) these 'add-ons" derate the unit from the basic capacity and results in a NET lift capacity.
Of course, there is the possibility attachments may have been removed from the unit & the data plate never up dated (a very common thing).

If you are looking to buy from an independent dealer or auctioneer on-line they may not be able to give you correct answers as they have no direct lines to the manufacturer.