The CBD:THC ratio of any hemp variety is a result of its genetic background (or genotype). The ratio is calculated as an average from a batch of plants. CBD and THC levels climb over time, and often spike close to harvest. It is always possible for a hemp variety to test over the federally legal limit of 0.3% total THC as the plant nears maturity. While environmental variables may affect the final percentages of CBD or THC, the ratio for a variety remains relatively constant when harvested at the suggested maturity. 

You can always ask a seed company for their COA (Certificate of Analysis) to validate the advertised cannabinoid profile.