The difference you note is a common problem that arises from the “directionality” in which base-pairs of the nucleotides (id’d as A, C, G and T), that make up each SNP (segment) of the DNA strand that are identified in the SNP database, are read.
This directionality may be expressed as “orientation”. The problem is that over time, the human genome assembly is “rebuilt”, i.e. each release (to the public) represents a new “build”; and gene direction may be read differently from one build to the next.
The chip that processes your dna sample (such as produced by Ancestry.com) will likely transcribe all the nucleotide base-pairs (from the X and Y chromosomes) from the “forward” or “plus” (+) position, while the SNP database is built from research which sometimes identifies the base pairs from the reverse orientation … i.e. “reverse” or “minus” (-) position; i.e. from a prior build.
These differences also show up between DNA services, by the way. DNA raw data processed by Livewello may be read in some cases differently by Promethease, which for some SNPs will identify the phenotype (the nucleotide letters) from the reverse direction (anti-sense).
So effectively, the problem for us is one of a difference in direction in the reading of the genome. I’m not sure how these differences occurred but possibly they reflect changes in the scientific convention over time.
But what can we do given this discrepancy?
Since we know that in the nucleotide pairs (according to the Watson-Crick model), guanine (G) is paired with cytosine ©, and adenine (A) with thymine (T)… we can infer the following:
You should be able to infer (in homozygous cases) that CC in the “+” position is “GG” in the reverse, and AA in the (+) Position is “TT” in reverse (-) orientation; (and vice-versa for both cases);
while you could translate heterozygosity similarly: A > T (and vice-versa), C> G and vice-versa…
Keep in mind that I’m neither an academic nor otherwise expert on this topic!
P.S. I just found a good page giving more on this: https://www.snpedia.com/index.php/Orientation . (If I’d read that first I’d just give you the link instead of writing the above.)