[Donald Rumsfeld Fan]

The reason involves the strong three-nucleon forces (a nucleon is either a neutron or a proton) within each carbon-14 nucleus. It's all about the simultaneous interactions among any three nucleons and the resulting influence on the decay of carbon-14.

[SunkenCiv]

[dead link]

The Radiocarbon Revolution
A Chemist Answers the Key Question: How Old Is It?
by R.E. Taylor
http://www.discoveringarchaeology.com/0699toc/6cover12-chemist.shtml

[snip] Radiocarbon dates differ from actual calendar years because, it turns out, the atmospheric concentration of C-14 is not constant, as Libby had assumed, but varies according to changes in solar radiation and in the intensity of the earth’s magnetic shield. Radiocarbon years can be “calibrated” to reflect calendar years, however. For instance, 12,700 years B.P. is equivalent to about 13400 B.C. [/snip]