On the difference of Starting Fajr / Dawn and Starting Isha / Dusk

Regards for higher latitude which fajr and Isha begins when Nautical twilight (-12°) even Civil twilight (-6°) in the summer, why there are too many criteria of beginning of Fajr (e.g. SIHAT Kemenag RI -20°, JAKIM Malaysia -18°, MUIS Singapore + MUIB Brunei -20°, Egyptian Geological Survey -19.5°, Muslim World League of Karachi -15°, Umm al-Qura KSA -18.5° etc) and Isha (Sihat Kemenag RI, JAKIM Malaysia, MUIS Singapore, MUIB Brunei -18°, Egyptian Geological Survey -17.5°, Umm al-Qura KSA +90 min. After Magrib (Ramadan +120 min. After Magrib), Muslim World League -15°, etc)? Why OIC not standardize and unified the criterion as OIC choose Turkiye Recommendation for Unified Global Hijri Calendar (as follow by Muhammadiyah, ECFR, FCNA and Turkey-follower countries)? New research from Tono Saksono prove that true fajr (fajr sadiq) begins near Nautical twilight. I've been asked this question in Astronomical Stack Exchange related on the affects of Light Pollution towards beginning of Twilight scientifically.