The 14C fraction of carbon varies. The cosmonenic production changes (it is about 5 kilogram 14C per year) and the reservoir (the amount of carbon) changes. For instance by burning fosil fuel the amonunt of carbon as CO2 in the atmosphere increases and the 14C/12C ratio drops. This is the fossil...
I think the the fractionation is about 2% (15 to 27 per mil). For 14C it is twice as much. In Radicarbon dating (and other 14C applications) , the 13C/12C ratio is used to correct for this effect.