Why does gluten cause some people abdomen discomfort when the blood work for celiacs disease was negative?

Unclear: This is a very unclear situation. There are two types of well described sensitivity to wheat. They are true wheat allergy and celiac disease. Both of these can be tested and clearly diagnosed. Non-celiac, non-allergic "gluten intolerance" is not well understood.

Variable: This question is difficult to answer because it really depends on what is being eaten. In general some people "respond" to limiting gluten despite not having the disease because they are now paying attention to food labels. Gluten containing foods tend to be highly refined or processed. So by avoiding these one is eating more real food.