Who is Annie McMillan?

While surveying Ames Municipal Cemetery in Ames, Iowa, looking for Willis & Elsie Gammon, I came across this confusing headstone for Annie Witherell Gammon, wife of William McMillan, who lived from April 28, 1850 to August 26, 1901. 

I cannot figure out who this woman is! It doesn't help that I don't know which of her names might be her maiden name. She could have been born a Witherell and married a Gammon before William McMillan or maybe Witherell is her middle name and Gammon her maiden name. She could have even married both a Witherell and Gammon before McMillan (probably not, though, she was only 51 when she died). 

She must be related somehow - she's a Gammon! Witherell is also suspiciously close to Wetherill, a name I know very well from a completely different branch of the family. One of my 5x-great-grandmothers was Mary Wetherill (m. Earl). Now, she is in the records as "Mary," "Polly," and "Anne," but she died in 1853 so this cannot be her stone, even leaving the Gammon/McMillan confusion aside. 

One day, maybe I will figure her out. Until then, she will remain an enigma.