Incidents may be an autobiography, but it's also a mythic account of Linda's rise out of slavery and triumph over evil. In that sense, Linda herself is a big, fat symbol: of triumph, of determination, and of the American spirit.
Even the title of the book tells us that she's more than just an individual person. "Incidents in the Life of a Slave Girl" strips the story of personal details and makes the slave girl of the title a symbol for all slave girls, everywhere. Linda's struggles are no different than any other girl's struggles.
But, of course, they are, because Linda manages to persevere her way out of slavery. Her story does not end "in the usual way" (41), because Linda is not just a symbol of an oppressed and degraded slave girl: she's a symbol of an American icon, the go-getter who pulls herself (or himself) up by her bootstraps to win respect, independence, and fortune. Sure, Linda's fortune might be her children rather than a pile of cash, but it's still a pretty good ending.