23andMe results indicate SNP positions and alleles based on the NCBI human genome assembly. Both the raw data as well as site features and reports use NCBI Build GRCh37 assembly.
While the reference human genome has been “finished” since 2004, it contains a small number of regions of unknown and/or incorrect sequence. When these inconsistencies are discovered, the reference genome is updated to reflect the more accurate genome sequence. Each of these updates is called a “genome assembly.”
The possible SNP genotypes reported in the Browse Raw Data feature might not match what you learn about the SNP from other sources. This is because every SNP can be represented using either of the two DNA strands (each chromosome is composed of two strands), and this representation will often differ from database to database or publication to publication. 23andMe always refers to the variant observed on the "plus", or forward strand of DNA. For more information, check out how 23andMe reports genotypes.