Have you ever seen a litter of kittens when they are all different but you know that biologically they all have the same parents. The genes from your parents combine very randomly with many many genes affecting different traits - sometimes the genes for a particular trait (say albinism) come from grandparents or great-grandparents. There are millions of molecules involved, all interacting in a systematic yet chaotic way and they follow quite complicated rules. Chances are, you and your siblings are from the same parents. Chances are also that you're not. The only way to know is to have a DNA test.

I have a very high portion of unassigned ancestry, 60%, in conservative view estimates in my 23andme ancestry dna test. I'm African-American, so, this may be a typical percentage for African-Americans in the conservative view estimates for 23andme. I emailed them and they mentioned that the unassigned is for multiple reference populations of European, Native American, and African ancestry that could not be assigned to a population. I took the 23andme v4 chip test, their current updated dna test.

Is the 60% in conservative estimates more likely Mestizo, Native American, or African, since the multiple reference populations of unassigned results in reference to European, Native American, and African?

No, it is a product sold to entertain people, purely as people are now interested in genealogy and some people think that they can have a DNA test and it saves them doing any research, however if you wish to know who and where your ancestors are from the ONLY way is to do the research.

...and YOUR ethnicity is YOUR cultures, customs, affiliations and associations in the country YOU were born, raised, live in and in YOUR family home, so the food YOU eat the language YOU speak, it is not anything to do with genealogy research or your ancestors or where they once came from, YOU are not them

Family Data Collections are taken from user-submitted data. I estimate them to be 75% accurate. AGBI is reputable. I have never used the Millennium File.

No genealogy record collection is 100% reliable, due to human error, faulty memory and infidelity. FamilyTree DNA advises customers that 2% of the people they test turn out to be surprised, due to hidden adoptions and infidelity.