The percentage is based upon the percentage within that companies database that’s why each result will be different.
Not sure what you mean but...
Your sample, for ethnic regions purposes, is compared to company stock samples. It is NOT compared to a database the company holds from its other customers. You are not being compared (for ethnicity) to everyone else.
The company population 'region/ethnic' stock samples can be very small, as few as less than a dozen samples for a region in some cases.
How can they possibly know the difference between ancestors from England, Ireland, Scotland, Wales, Northern France, Belgium and Demmark from hundreds of years ago, when they only compare your sample to a few dozen other 'stock' samples? And all these places shared more or less the same ethnicity, Western European, anyway?
A more honest ethnicity result would say something like
'Your ethnic origin is 85% North Western European, 15% uncertain'
But they wouldn't sell as many tests that way, especially in the huge American market
The whole 'ethnicity' percent side of it is a rip off and people are spending money, money they might need for other things. £100 is a lot to fork out in one go for a lot of people.