So there are three colors in the chart, representing three match levels:
Gray - no match
Purple - half-identical (meaning they share segments from one parent or the other)
Dark purple - completely identical (meaning they share segments from both parents)
I think I'm beginning to really see why siblings only share 50% of their DNA on average - the probability of sharing the exact same data from both parents all across all 23 segments is really low. My mom and her brother have a little over 3000 cM of half-identical segments, but only 1200 cM of fully identical segments. Chromosomes 18, 19, and 20 are almost all identical all the way across, but all the rest were only fully identical in bits and pieces.
Chromosomes 21, 22, and X all have no fully identical segments. With 21 and 22, that's due to the random recombination of DNA, so they could have had fully identical segments there, they just happened to not get them. With the X, they couldn't have any fully identical segments, as my mom has X DNA from both her mom and dad, while her brother only has X DNA from his mom.
So this was really interesting for me to see, and helped me see how DNA inheritance among siblings works. I've known in my head for so long how much siblings should share, but it's really eye opening to see it work in action.