But check the actual source and:
"Among women who had unintended births in the United States in 1998–2002, about 40% were using contraception"
So that's very misleading reporting.
Note that they tracked women for 4 years - track someone for their entire life and the numbers who had at least one child while on birth control at some point in their life will rise.
Contraception just isn't effective enough to eliminate pregnancy, it can only function to reduce fertility (number of children).
A typical couple using birth control through their entire life will still have one or two children - which is the average (more or less) of people in the US who choose to have children.
The only function of birth control is to reduce fertility from around 10 to around 2.