Here's why:

* Carbon-14 (¹⁴C) is a radioactive isotope of carbon. It decays over time at a known rate, called its half-life.

* Carbon-12 (¹²C) is the most common isotope of carbon and is stable (does not decay).

* The ratio of ¹⁴C to ¹²C in a sample is measured.

* The age of the sample can be calculated by comparing this ratio to the known ¹⁴C/¹²C ratio in the atmosphere when the organism was alive.

So, while radiocarbon dating relies on the decay of carbon-14, it is not a comparison between two substances. Instead, it's a measurement of the proportion of one specific isotope (¹⁴C) within a sample.