I believe this phenomenon is best viewed through the lens of the interactive activation model for word recognition (McClelland & Rumelhart, 1981), which was put forth to explain a similar phenomenon, namely the ability to more easily recognize letters within a word as opposed to to letters not comprising a word.
As the image below illustrates, this ease of recognition comes from the top down influence of the known word concepts on the possible identities of the letters.
Now, to tie this back into your original question, we can view the task of remembering a password as one of converting a stored concept into a character string (i.e. something like the interactive activation model without the bottom up line component). If your concept is "strawberry", then there's a strong top down influence to choose 's' as the first character of the password, since that's the most common first character of "strawberry". Now many have formed associations between 's' and '$', so perhaps the latter might get some response, but not nearly to the extent of the actual letter, since letters are far more commonly used within words than symbols are.
McClelland, J.L., & Rumelhart, D.E. (1981). An interactive model of
context effects in letter perception: I. An acccount of basic
findings. Psychological Review, 88, 375-407.