My father's grandfather was born as one of a set of twins to a woman named Margaret, back in 1886. Margaret was a widow with other children at the time of the birth of the twins. She kept one and raised him, and gave the other (my dad's grandfather) to the other family to raise.
This is the story we've always been told but it's never been proven. My great aunt told me a bunch of details that she could remember, and I decided to research to prove it. Recently I found the paper trail and a great many details all fitting exactly with what she told me.
There is a lot of evidence pointing to Margaret as being my dad's great grandmother. I searched and found a couple of trees with her in them, contacted her direct descendants, and asked one to do a DNA test. He is also a great grandson of Margaret, and readily agreed and was very interested and happy to do so. He and my dad would be half second cousins.
The results have just come back and indeed, he is a match to my dad, but the number of cM are below the lowest number of 30 in the range of about 30-215 for half second cousins.
According to Gedmatch, my dad and the tester share two matching DNA segments, one on Chromosome 3 at 7.1 cM and 928 SNP & the other on Chromosome 11 at 8.5 cM and 806 SNPS for a total of segments > 7 cM = 15.6 cM. The paper trail is very clear and lines up exactly with what our family has always believed, and there is a cousin relationship.
Is it possible that this is just an anomaly of some kind?