Why are parent-child relationships always exactly 50%, without a range? (Note th

Why are parent-child relationships always exactly 50%, without a range? (Note this chart, like most DTCGT, is working with autosomes, aka chromosomes 1-22.)
Can you tell from DNA comparison who is the child and who is the parent? (Companies might use other user data collected at sign up to decide for sure.)
Why is there a range around the relationship between full siblings?
Are the full siblings on average as closely related as parent-child?
What are ¾ siblings? (Why does the chart say they are a “combination of half siblings and first cousins”?)
Is your answer in the previous question the same thing as incest, or is it a different way that families get complicated?
What are double first cousins? Is that incest, or is it a different way that families get complicated?
If you find fully identical regions in comparing two people, could you rule out half-siblings who share only one parent in common, assuming that their other nonshared parents were not related?
What second-degree relatives are as closely related as a half-sibling, and indistinguishable from looking at genetic relatedness alone?
How could you (or a DTCGT company algorithm) use age to try tell the difference between the answers to the previous question? Is that always effective or certain?
Now, read through the narratives of (real life) scenarios where people discovered surprising information about close-relative relationships as a result of direct-to-consumer genetic testing: attached
If any of the scenarios is too personally sensitive, feel free to skip it. You need to read enough of these to be able to compare/contrast and find themes, which you can do if you opt out of some.
11. Did people in these narratives overall seem prepared or unprepared to find these relationships when they ordered the test? Did prior education seem to help people expect these results?
12. How were the unexpected relative findings initially received by the people who ordered them — any common reaction patterns?
13. Do you note any differences between shorter-term and longer-term reactions to discovering the unexpected genetic relationships? (Consider the souring of relationship in scenario 8 in particular.)
14. Does the situation uncovered by DNA testing always involve someone deliberately hiding information?
15. When you compare an individuals’ two redundant chromosomes to each other, and there are long stretches of having inherited the same alleles both maternally and paternally, what could that possibly mean about the nature of the individual’s parentage?