Because if GM is such a predictable, precise science, then you should be able to produce the same thing again and again. But you can't. Regarding our potatoes, even after many lines were thrown out, the ones which we retained were still all different from each other. Even though they all came from the same pot, using the same genetic construct, and were grown in identical conditions. They must not be different.
Causative logic says that they ought to be the same.
That is for me the most worrying aspect. These people can only live their lives on the basis that they know which foods to avoid. But there are thousands of other foods that can cause serious allergies but which are not classed as known allergens. On top of this, there may be new toxins or allergens in GM foods that are not spotted because they are not looked for. But what you are saying means that even if you test three potatoes and find that they do not cause an allergic reaction, a fourth potato of the same kind, produced by the same technique, could cause a toxic or allergic reaction.