It's based on each company's particular sample collection. MyHeritage has the fewest samples so their results are the most inaccurate. But even with AncestryDNA, which has the highest, they're basing their results off of thousands of samples out of hundreds of millions to billions of people overall. For instance, 23andMe might have 4600 samples from Sub-Saharan Africa, but SSA has 850 million people. Nigeria alone has 200 million. It'll never be exact without testing every last person on the planet. It's more like polling. You ask a few people to get a consensus of the greater whole. But each company polled different people. So just like political polls they aren't going to all line up perfecting. A Pew Research poll will vary slightly from a Fox News poll which will vary a little from a CNN poll. But generally speaking they're all in the same ballpark. It's the same with these DNA ethnicity tests. If you do a test with 23andMe they have a slider that adjusts for confidence levels. It's defaulted to 50%. That's the results you get. If you slide it to 90% you're mostly going to see broad categorizations like "Broadly Sub-Saharan African" or "Broadly West African" take up the lion's share of your percentage. Specific areas like Nigeria, Kenya, etc. will drop really low to numbers like 1.2%, 0.5%, etc.
