Ninth child of John the Younger and Margaret

I have run across an interesting problem in doing my biography of John the Younger, who married in 1697 and became President of Harvard in 1708. No one has done a biography of him before but several people have written fairly detailed sketches.

Some of the sketches attribute to him and his first wife, Margaret Rogers Berry Leverett, nine children, while others mention eight. Margaret herself mentions eight in a letter that she writes in 1717. The eight that are uncontroversial I'll list in order.
1. Margaret b. 30 Sept. 1698 d. 22 Nov. 1702 (4 y. o.)
2. Sarah (Wigglesworth) b. 12. Nov. 1700 d. 9 Nov. 1727 (lived to age 27, married, no children)
3. Mary (Rogers) b. 29 Oct. 1701 d. 25 June 1757 (married twice, ten children, died in Ipswich)
4. John b. 26 Sept. 1703 d. 31 Oct. 1704 (13 mo.)
5. Payton b. 4 Aug. 1704 d. 7 Dec. 1704 (4 mo.)
6. Margaret b. 31 July 1705 d. 16 June 1716 (11 y. o.)
7. Ann b. 5 July 1708 d. 30 July 1708 (25 days)
8. John b. 21 June 1711 d. 4 July 1711 (13 days)

The Leverett Memorial, which contains the letter from Margaret, does not try to list out these names and in fact I have never found a complete list of his children. It is interesting because it is pretty thorough in other ways, listing out children of other ancestors, but does not touch John's.

The ninth child is apparently a Mary Leverett, who died in Cambridge in 1699. People who were scooping the Cambridge death records would have found seven of these eight, but would have found Mary as well, since she was listed there along with the other Leveretts, plus a few more that came along later in the century.

The question is, where did this Mary come from? She could have been John's sister, who was born probably in 1674, and therefore would have been 25. Deaths at age 25 were common in those days and in fact there weren't many Leveretts around, not many came from the South, not many new ones had come over from England.

I was working on the plan that Mary, John's sister, was the Mary who married a Johnathan Moulton in Ipswich later in the 1700's, under the same assumption that not many had appeared, not many had just showed up from somewhere in the South. But some Mary came from somewhere, and died in Cambridge. More later.