The twenty-two letter alphabet gives three signs to the s-like sounds, singaat, tsaadiy and shan, and asks one of them to do the work of two. The ancestor language had more distinctions in this area than the alphabet has letters for, and the languages that used the alphabet had already merged some of them, so the signs are a compromise made once and inherited ever after.
The consequences run through the whole family. The letter that writes sh is read as sh in some words and as s in others, and the square Hebrew script eventually invented a dot to mark which, a device the Samaritan script does not have and does not need, because the reading tradition carries the information. Arabic, which fixed its alphabet later and for a language that had kept more of the distinctions, writes the same territory with a different set of letters, and the correspondences between an Arabic word and its Hebrew or Samaritan cousin in this area are regular but not obvious.
What this means for anyone reading the letters as evidence is a caution. The alphabet is not a phonetic transcription and was never meant to be one. It is a set of signs adequate to write a particular language at a particular moment, retained afterwards through several centuries of sound change in more than one community. Reading it as a direct record of pronunciation, in any period, produces confident conclusions the evidence does not support.