On its initial release in December 1960, the U.S. release on the Argo record label was titled "I Don't Know Why". However, about ten weeks later, Argo announced that due to confusion arising from the song being mistaken for the 1931 song called "I Don't Know Why (I Just Do)", they changed the name of this song to "But I Do".[2] The UK release on the Pye label and the Australian release on the Coronet label were both titled "But I Do".
Source: Wikipedia