Yeah I don't think allergies depend on external factors that much. My mother and I have pretty equivalent childhoods even down to same locations and she's wrecked with allergies and I have none, yet my health is worse overall (despite me having a healthier lifestyle).
While I believe some exposure helps build resistance ultimately your overall health will be influenced by stuff beyond your control AND your lifestyle/activities combined.
So are all of my siblings though, so there might be more of a genetic component to this.