The Tetragrammaton is one example of the phenomenon called "k'ri u'k'siv" (קרי וכתיב), where the word written in the text is not the word spoken aloud by the reader. In printed Bibles, the k'siv, or word written in the text, remains in the main text, while the k'ri, the word actually read is put in a textual note. In older printings with manual layouts would put the k'ri in tiny letters in the margin with a note in the main text to direct the reader there. Because the letters were too small to add vowelizations, they would be put in the main text with the ksiv, and it was the readers job to apply the vowels in the text to the word in the margin.
In the case of the Tetragrammaton, the word is pronounced adonai, with the vowels patach, choilam, kamatz; i.e. ah-oi-aw. This would be printed with the letters of the Tetragrammaton, but not meant for those letters. Because it was so common, there would not be a note every time. So one who did not know better would naturally apply the ah-oi-aw to the letters j-h-v-h, rendering Jahova, but it is nothing more than a mistake.
The true pronunciation of G-d's name was only said aloud by the High Priest on the Day of Atonement. Since the Destruction of the Temple it is only known to those deemed worthy to learn it.