The grey histogram shows possible ages for the sample (the higher the histogram the more likely that age is).The results of calibration are often given as an age range.
See also ORAU's Explanation of Radiocarbon Results.
It has to do with the advent of carbon dating techniques.
Radiocarbon measurements are always reported in terms of years `before present' (BP).
For two important reasons, this does not mean that the sample comes from 3619 BC: Many types of tree reliably lay down one tree ring every year.
The wood in these rings once laid down remains unchanged during the life of the tree.