It is basically the difference between knowing and applying what one knows. This is the proverbial gap between theory and practice, or between knowledge and wisdom: one may have learned something, but does not use what is learned.
When one says something, but does another thing: well, this is almost irony in action–except that in irony, the element of awareness about what one does, is normally the informing stimulus for the utterance.
A simple comparision of two very similar methods of writing.