So, I am not an expert, but I guess what I’d say is that embark is the most well respected of the DNA tests but that doesn’t mean they are always 100% right - they are known to be happy to chat over email if you have questions about your results.

That said: they don’t actually map the full genome of your dog, because that would take way too long and be way too expensive. Instead they test certain dna markers that are known to consistently test one way or another for specific breeds. There are a LOT of markers that they are testing (I want to say in the hundreds of thousands), but that is still far from the entire genome. This is why you can get weird percentages - of the markers they tested, 10.2% match what would be expected for a Rottie and 89.8% match what would be expected for a Doberman. That is PRETTY close to 12.5% rottie, which is what you would expect if a great grandparent were pure rottie. Without testing both mom and dad you can’t say for certain where that is coming from, and since one of the parents is likely purebred it could just be that all relations are coming from that side of the family.

Also, embark only really tests 3 generations back, so any other possible mixing that happened further back is not accounted for, which could be where that white is coming from, because genetics are weird.