The smaller the match, the more distant the relationship. So it can be harder to work out where the smaller ones fit in. Phoenix at 8cms that could be an error.
There is also IBD vs IBS ie identical by descent vs identical by state. The first is obvious but the second one is just co-incidence. The smaller the match, the more likely it is IBS.
Libby I think the best way to work out your issue is to look at the dna through a chromosome browser, which ancestry does not have, to see where and how big the other side actually is.
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Toni 
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