Yup, it is a coincidence, but that is what the numbers are: there are exactly 100 more women listed in the index to the 1850 census on Ancestry.com named Sabina than there are women named Sabrina. It's not a mistake, just a random fact. :)
That makes both Sabina and Sabrina rare. There are 2,088 women named Wealthy; 19,907 named Almira; and 1,343,878 named Mary in the indexes to the 1850 census.