Here's the answer that I received from, Family Tree DNA Help Desk:
Different Ethnicity Reports
Each company will have different academic reference populations that they compare your data to. Because there are different reference populations, and because each company also has a different algorithm for calculating those percentages, the results will vary from company to company.
Each company also tests different autosomal SNPs, so we're each looking at a different data set for you to begin with, which will further cause differences. To see how many SNPs each company has in common with the Family Finder, please view the link below from the International Society of Genetic Genealogy.
Autosomal SNP comparison chart - ISOGG Wiki
Autosomal DNA Inheritance
Autosomal DNA is passed down via random recombination. This means you do not inherit a perfect half of what your parents have.
Autosomal DNA, including the sections that allow us to interpret ethnic percentages, is passed down in sections or chunks. When the DNA randomly recombines, it's recombining segments of SNPs instead of individual SNPs. For example, instead of getting an A from mom and then a T from dad and a C from mom and a G from dad, you're getting ACACACACACACACACAC from mom and then TGTGTG from dad. When it all evens out, about half of the segments came from mom, and about half of the segments came from dad. This is how you can end up with a child that phenotypically resembles one parent more than they resemble the other.
However, where this can all show up most glaringly is in the myOrigin results. While mom might be 50% Central Europe, 20% East Slavic, and 30% Scandinavia—that autosomal DNA doesn't just split in half perfectly when it gets passed down. It gets passed down randomly. So the child can inherit 5% Central Europe, 15% East Slavic, and 30% Scandinavia from mom—or any other random combination of these percentages. The same goes for how dad's percentages will be passed down to the child. Compounded across multiple generations, this kind of randomness can explain why myOrigins results are often not what we expect them to be—you simply don't inherit autosomal DNA from all of your ancestors, and we cannot connect you to populations with which you didn't inherit shared DNA.
Because of this, autosomal DNA testing can prove ancestry from a specific region or ethnicity, but it can't disprove it.
You can read more about ethnicity testing at the links below:
Ethnicity is just an Estimate
Ethnicity Testing: A Conundrum
Ethnicity Results, True or Not
Ethnicity Percentages Changing
MyOrigins ethnicity percentages are always updated to include the most up-to-date scientific knowledge about autosomal DNA. MyOrigins 2.0 had 24 population clusters and myOrigins 3.0 has 90. This means that based on the latest scientific knowledge, we were able to further refine the results.
If your percentages changed that is because the markers associated with the previous population clusters are no longer associated with them. Based on the latest scientific knowledge, the markers are now associated with the population clusters you see in your myOrigins 3.0 results.
Genetic genealogy and population genetics are very recent fields of scientific research, so we are constantly learning new things. That is why we continue to update our population clusters. Also, please be advised that autosomal DNA markers are not necessarily associated with places of birth or genealogical records. The inherited markers can be hundreds of years old.
See the articles below for further clarification:
Making Sense of Ethnicity Updates
Don't Like Your Ethnicity? Wait 5 Minutes
Ancestral DNA Percentages
My conclusion on this is, if I am correct each child may/will get different amounts of DNA from each parent. So each child will have different percentages of certain DNA. No two childs DNA is the same percentage of an ethnicity.