As I was out walking my pupster. i'm recalling that many of the 8 lug SW/DW rigs do not come std with a spare tire in the past. Did you order a spare, thats 60-80 lbs right there!!
Did you get one of the very basic option packages with AC, upgrade radio etc? thats potentially another 40-60 lbs of added weight, lost payload to your truck brochure payload!
Did you put after market spray on undercoating on your truck? Line-x on the steel bed? extended camper mirrors?
If the backup camera is an option.....that's 5-10 lbs of lost payload!
I'm sure that 100 lbs of missing payload are option(s) you are not including in your numbers.
Marty