In 1887 a widowed relative and her child immigrate to the United States. In 1888 that widow marries a United States citizen. At that point in American history, the widow and her child are now United States citizens. As a woman what rights she had at that point in time are another matter entirely.
But if the researcher encounters a later census enumeration where the date of immigration and the date of naturalization for the grown child look a little “odd” because they are one year apart–that’s the likely reason for it.