This is interesting.
Younger DS tested negative to milk when he was younger, despite clearly reacting within minutes with bright red flushed cheeks which turned into nasty eczema hours later. His allergist's motto was that he treats people, not test results, and to consider him allergic to milk - which he outgrew shortly after his 5th birthday. (He started being able to tolerate baked milk around age 3).
This was around 20 years ago, but if I remember correctly he said that current tests didn't test for all the proteins in milk, and that he could be allergic to one of the minor proteins that weren't included in the test. I wonder if this could also be the case for other foods.
Thanks for sharing!