Why is harappan script called enigmatic?

Over the past century, archaeologists working in the Middle East have time and again excavated seals bearing what they call “Indus-style inscriptions”, complete with the script and the usual unicorn or bull. However, such Indus- or Harappan-style seals discovered “overseas”, so to speak, displayed a different shape as well as craftsmanship, being shaped either as circles or cylinders which were “rolled over wet clay rather than pressed upon it.” At times, such seals have also been discovered from within the defined boundaries of the Indus Valley Civilization – or, shall we say, Meluhha, as most modern scholars agree the contemporarily-named Indus Valley Civilization was called at the time of its existence – such as the “Gulf seal” discovered from Lothal, Gujarat. All of these discoveries give rise to many questions: did the people from the-then Dilmun and Magan Civilizations – as the civilizations from the modern-day Bahrain and Oman were respectively known – produce those seals indigenously? If so, did they understand the Meluhhan language? Or did the Meluhhans themselves make different seals for trade-items being sent to different places? In which case, too, the question remains: could the people from neighbouring civilizations understand the script? If they could not, then why did the Meluhhans send them these seals? Or, if those civilizations produced them indigenously, why so? Answers to these questions might hold the key to the decipherment of the Indus Script, by aiding archaeologists in the discovery of a bilingual text: the Rosetta Stone of the Indus Valley Civilization.