Who the hell actually knows the SHU of the food they eat?
Very few; at most they know what is written on the label or on some website. The standard (American Spice Trade Association) way is that Scoville heat units are a measure for the concentration of capsaicin in chili peppers, in terms of dry weight. Dry weight is about 10% of the fresh weight of chilis. For chili sauce, they rarely specify whether the Scoville rating is in terms of "wet" weight of the sauce, or on a dry weight basis - leave alone what the water content is of those sauces.
And especially for super hot chilis (Bhut Jolokia and so on), the numbers reported are for one particularly hot batch. That doesn't tell much abouw how much heat an average specimen, grown under average conditions, will get you, but you can be pretty sure that it will be less than the batch that was tested with the aim of getting into the Guiness book of records.