Jurisdictions are tricky things, particularly in colonial America. We are used to seeing towns associated with just one county and state. In British America, towns, plantations (areas governed by a merchant company or the forerunner of a colony), and even colonies themselves were fluid.
For example, the town of Dover started out as a fishing and trading settlement established by a group of Bristol merchants who named the area Piscataqua Plantation. Part of the plantation, Hilton Point (settled by William and Edward Hilton), increased in population and in 1633 it was purchased by a group of Puritans in England who promoted settlement there. In 1643 Dover became part of the Massachusetts Bay Colony (under Old Norfolk County which was later absorbed into Essex County). It wasn’t until 1680 that Dover came under the governance of the New Hampshire Colony. Dover was also known at various times as Bristol and Northam.
In a period of less than sixty years, Dover had at least three town names, was under three major governing entities (plantation and colonies), and thus multiple counties. Nantucket began as part of Dukes County, which was then part of the Colony of New York, where it remained until 1692. At that point, it became part of Massachusetts Bay Colony and its own county of Nantucket. When place names are given in posts, they will use the designation appropriate for the time of the source. Seeing Dover written “Dover, Piscataqua, British America in one place and Dover, Strafford, New Hampshire in another is not a mistake, but an attempt to keep the place designation accurate and to show what entities held the records when they were made.
Sometimes documents stay with the original governing body even if political divisions change. Other times the records, or duplicates of them, are placed with the new governing bodies. To truly canvas all possible record holders for Dover and Nantucket, multiple town, county, colony, and state records had to be searched.