In Bengal, the Gayatri (Savitri) Mantra always starts with OM and ends with OM. The mantra is thus printed in the very famous book 'Ahnik-Krittya' by Shyamacharan Kaviratna Vidyavaridhi.
But so far as I know, in the other parts of India, the Mantra only starts with OM but does not end with OM; the rest is identical though.
What is the reason for this difference?