When a vendor reports nutrition values for a 100g sample, there should be 100g of nutrients reported. Similarly, when you enter nutrition information for a 100g sample of an item, the total weight of the nutrients should equal 100g.
If this is not the case, the most common reason is that the values for water and ash were not reported. If you have water and ash values and there is still a large deviation, call the vendor for clarification.
With a few exceptions the 100g is the sum of the following nutrients:
These are the nutrients that are reported in grams.
Vitamins are reported in milligrams or micrograms and usually do not provide a significant contribution to the total nutrient weight. Minerals are included in the Ash total.
Note that the USDA standard nutrition reference database is very good in this regard. The vast majority of items add up to exactly 100g.