These are my suggestions:
It is assumed that some idioms (like the previous examples) have acquired a new meaning, one different from the original. They have become standardized metaphors to imply roughly the same ideas. These new versions of the idioms may have continued to be used long after the orginal meanings lost all meaning to most people. That is to say, people have continued using some of these expressions even though they did not know how they came to be nor the implications that they previously had.