Qitzur Shulchan Arukh – 67:6
The neder doesn’t come into existence unless his mouth and heart are the same. However, if he made a neder by accident, that his thought wasn’t like what came out of his mouth, or if he contemplated a neder in his heart but it didn’t come out of his mouth, it is not a neder.
That an unintentional oath, a misspeech, isn’t binding, shouldn’t surprise.
However, it is very Jewish that the reverse — an unexpressed thought — lack the same reality as an expressed one. In general, Judaism focuses on how we impact ourselves. The thought becomes more real and more ingrained once it’s acted upon.