The key thing to understand is that, yes, you do get 50% of your genes from each parent, but that does NOT mean that you get 25% from each grandparent. For example, your mom could have given you more of her dad's genes than her mom's genes, and that can influence ethnicity estimates. Another thing that is does NOT mean is that siblings get the same 50% of each parents' genes as every other sibling. Otherwise, all siblings would be twins, right?

Let's say that you had a maternal great-grandmother that was 100% indigenous Mexican and married someone that was 100%, I don't know, Italian.. They have a child, your maternal grandfrather, who is now 50% indigenous Mexican and 50% Italian. Now let's say that he marries your mom, who is Japanese or something. Now your dad is going to give you 50% of his genes, but there's absolutely nothing stopping him from giving you mostly his Italian genes, or mostly his indigenous Mexican genes. In theory, in this situation, you could show up as anywhere from 0% to 50% indigenous Mexican, with an expected average around 25%. Yes, even though, you have a full-blooded indigenous Mexican ancestor, you could be showing up as 0% (0% is an extreme example, but it illustrates the concept).

So, how much of your DNA does a person actually share with an aunt or uncle? Between 0% and 50%, with an average of 25%. Same with a grandparent.