How Is This Possible?

[mgeneas]

My granddaughter has a match on ancestry to David 40 cm over 2 segments.
Her full sibling is 12 cm over one segment. I am fine with that - but

Their mother and aunt share 30 cm  and 20 cm respectively with David.

How can granddaughter have a bigger match than her mother to him?

[brigidmac]

I have a similar example ..I think the granddaughter  may have distant  links to David ...on paternal side too ....

Haven't proved it but someone is coming up 4th cousin to me 3rd cousin to my
Mum yet his aunt is only 5th or 4th relation respectively

[diplodicus]

When comparing lengths of matches, you need to eliminate those pesky "pile-up zones"; the strands of dNA that all modern humans share. Just Google "Pile up zones" and prepare to lose another couple of hours of your life!!