Looking for the Super-Genome. -And it ain't found

Hey jar, I'm a newbie and have a newbie-type question. I only skimmed over the topic, so I'm not even sure this question hasn't been asked yet. If it has, I apologize.Now, you talk about mitochondrial DNA and sub-groups and whatnot. What limits the children/grandchildren/etc of Eve to having the same haplogroup mitochondrial DNA from the mitochondrial Eve? Why can't their mitochondrial DNA be a sub-group of hers?

What about the article "Calibrating the Mitochondrial Clock" by Ann Gibbons, which basically states that Mitochondrial DNA, in some cases, mutate much faster than expected?

"Some of the DNA from the tsar's mitochondria--cellular organelles with their own DNA--didn't quitematch that of his living relatives. Forensic experts thought that most people carry only one type ofmitochondrial DNA(mtDNA), but the tsar had two: The same site sometimes contained a cytosineand sometimes a thymine. His relatives had only thymine, a mismatch that fueled controversy overthe authenticity of the skeletons."The question of the tsar's bones was finally put to rest after the remains of his brother, the Grand Duke of Russia Georgij Romanov, were exhumed; the results of the DNA analysis were published in Nature Genetics in 1996. Like the tsar, the duke had inherited two different sequences of mtDNA from their mother, a condition known as heteroplasmy."The duke and the tsar lived less than 150 years apart. 2, maybe 3 generations sounds reasonable, and even if it were a couple more, would that matter?