As I understand it, Ancestry has the biggest database of dna results as it is the most popular. As more people have their dna tested the database gets bigger. The results change to reflect the more recent data results. When I first joined Ancestry in 2017, I was shown as 92% Irish but also had Finnish/Russian, Northern Europe, and Polynesian. As the results have been updated, the Ancestry DNA people can pinpoint with greater accuracy to certain areas and the older or more ancient DNA vanishes up the list. My Ancestry results now show me as 98% Irish Scottish with a smattering of Welsh and English, which shows that although people in Britain can claim certain blood lines most of us are a mix of many. My Heritage has me as 98.5% Irish and a dash of the Levant, which is the Middle East. DNA testing with the known and larger companies is generally correct. However, DNA can prove that sometimes a supposedly known relation, is not a genetic relation and that can be a shock. I am an adoptee and found my birth father's family and a half brother through Ancestry. I would never have found my brother or my birth father's family without the DNA test.