These Bristol probate cases were not labeled with official case numbers, so an index had to be created from the microfilmed file papers. The resulting index is mostly alphabetical. However the file papers were not microfilmed in strict alphabetic order and case numbers were assigned as the cases were examined to create this database. Thus, the statement “mostly alphabetical.”
For a time, the borders of Massachusetts and Rhode Island (where Bristol County is located) shifted considerably as the two states swapped parcels of land. Bristol County, MA borders Bristol County, RI. Wikipedia explains this is one of ten instances in which adjacent counties in bordering states share the same name, and the only example in New England. Other examples include Teton County in Idaho which borders Teton County in Wyoming and Kent County in Maryland which lies next to Kent County, Delaware. You can consult History of Bristol County, Massachusetts edited by D. Hamilton Hurd for more details on the border changes in the 1700-1800s between Rhode Island and Massachusetts.