Not sure , you should ask these people? But here's one explanation. ...."If a person is allergic, they may be best able to tolerate a specific dog, possibly of one of the hypoallergenic breeds. Dr. Thomas A. Platts-Mills, head of the Asthma and Allergic Disease Center at the University of Virginia, explained that there are cases in which a specific dog (not breed) might be better tolerated by a specific person, for unknown reasons. "We think there really are differences in protein production between dogs that may help one patient and not another," Dr. Platts-Mills said
That's very true, but it's also true that some breeds are worse culprits for a particular person than other breeds are. For me, all the oily coated out door hunting or water dogs are the worst. This can be SOMEWHAT alleviated by frequent bathing. (which those dogs typically don't get, just because of their size) My brother has a rescue hound from Kentucky. He's a neatnik, and that dog gets bathed weekly, just as most of us who have Havs and allergies do with our dogs. I'm LESS allergic to him than most hounds, but still more allergic to him by far than I am to any of the (many) Havs that I've handled.
Also, if it were specific to one dog, I would hae had more trouble when I went to pick Kodi up. We spent 2 days with his breeder (including sleeping there) and 11 or 12 Havs. If Havs were going to be a problem for me, I would have seen it there.
Allergy testing and studies like this can be useful in moving forward the body of knowledge we have about allergies IN GENERAL. I have found (living in a very allergic family) that they have less implication on an individual level. My so, who has severe (anaphylaxis which lands him in the hospital several times a year) allergies TESTS as being highly allergic to peanut butter. But he can and does eat that with impunity. With all their fancy tests, they can't tell us for sure what sends him into anaphylaxis.
As far as the early exposure helping... maybe for some people, but not for me. I was born into a house with a Pug, and that breed is one of the worst offenders for me in terms of allergies. It IS true that my body tends to adjust to specific animals I'm around all the time, if the allergy wasn't too severe to start with. For instance, I am less allergic to our own cat than I am to other people's cats. And this has been true of a long string of cats.
Allergies are a VERY individual thing, and from what I've seen, all generalizations about allergies are suspect.