This is a follow up to Probability of Specific Relationship from AncestryDNA?
I have an Ancestry match with two half-sisters (same mother, different fathers). The older one by 16 years matches me at 684 CMs and the younger at 461 CMs.
Using the DNADetectives chart, these suggest two possibly different relationships with me although there is a very narrow overlap.
How could two sisters with a common mother, match to me with such a gap in CMs?