No DNA tests tell you who your specific ancestors were.

For the overall ancestral testing they use the Autosomal DNA along with the X. 44 of your 46 chromosomes are Autosomes and you got Autosomal 50-50 from both parents but while you got 50% from your mother's side and 50% from your father's, there usually will be some bias in what you inherited from grandmother and grandfather on both sides of the family In other words you normally do not get 25% from each of your 4 grandparents. Actually you could have inherited anywhere from 0 to 50% from any one grandparent. How you inherited the bias will not be how your siblings inherited it unless you have an identical twin. So that means if you and a full sibling who is not an identical twin were tested by the same company at the same time, the results probably will not be exact.

http://www.dnainheritance.kahikatea.net/...

Also if you go to more than one company to have this type of testing the results will vary as the only thing companies can do is match you with population groups in their database. They each have their own database and therefore if one does not have certain population groups another has and vice versa, naturally it can mean a difference in your results with each.

It has also been reported if you go back to the same company and are retested again sometime later, the results might change as by that time they have received more population samples which can change the picture.

Now the real way any genealogy DNA test can be helpful is if you are into traditional genealogy work using documents/records and if the company you choose has cousins of yours going back several generations in their database and they notify you of those cousins and they also into traditional genealogy work, you can collaborate information with them. They might have discovered ancestors in records you haven't and you might have discovered ancestors in records they haven't..