When an organism dies (whether plant or animal) its intake of carbon atoms ceases.
The starting ratio of radiocarbon to stable carbon is locked in at that point. The purpose in each of these methods is to determine the ratio of radiocarbon to stable carbon in the sample.
The raw radiocarbon date of any sample can then be converted to true date by using this calibration table.
This calibration step eliminates any concern about fluctuations in historic radiocarbon to stable carbon ratios or decay rates.
Radiocarbon dating is a valuable tool to chronologists and archaeologists.