cM matches

[Phoenix]

Best Mate has seven different DNA matches to a mutual ancestor, via three different brothers, so I am happy that the DNA agrees with the paper trail.

However, the results are odd on one branch where two sisters, each with a daughter have tested:

Sister A 12 cM and her daughter 13cM
Sister B 6cM and her daughter 12cM

Sister B and her daughter both have an unweighted segment of 13cM
Sister A and her daughter have unweighted segment agreeing to their stated DNA.

Does this mean, in reality, that all four share the same 12 cM in common with each other and my friend? I find it disconcerting that Ancestry would have rejected sister B as having too little common DNA.

Or is it the case that the shared DNA is actually much less, and most of the segment is just noise?