Hi,

The NCBI link speaks about your query for the most part.

Please go through it, and generally the frequency of the ancestral Vs variant allele depends on your population or dataset too. G/C allele frequency in dbSNP can be different for you if you have C as your ancestral allele. But the orientation what they say is to make it matchable to most databases which is why it will not vary with builds but it might vary from a paper when compared to database. If UCSC has used the reverse orientation for its Ref, and other databases too, then dbSNP reports it in that form and frequency is calculated.

http://www.ncbi.nlm.nih.gov/books/NBK44476/#Reports.dbsnp_reports_rs10512248_allel_1