In To Kill a Mockingbird, why does Scout assume that Mrs. Merriweather is talking about Mayella when she says, "I always say forgive and forget"?

In Chapter 24 of Harper Lee's To Kill a
Mockingbird, Aunt Alexandra entertains a meeting of the local ladies'
missionary society.  This group includes Mrs. Grace Merriweather, Miss Rachel, Miss
Stephanie Crawford, and Miss Maudie.  Despite the fact that the Mrs. Merriweather speaks
extensively on the horrors of life among the Mrunas, a woman ("Gertrude") sitting near
her manages to turn the conversation to local concerns with a single
comment.

When Scout overhears Mrs. Merriweather mention
"sin and squalor" and "forgive and forget," as well as Merriweather's comment that
what

...the
church ought to do is help her lead a Christian life for those children from here on
out.  Some of the men ought to go out there and tell that preacher to encourage
her...,

Scout assumes that
Mrs. Merriweather is referring to Mayella Ewell.  The notion that Merriweather might be
referring to Helen Robinson never occurs to Scout.  The idea of judging Helen based on
unfair and unjust racial stereotypes is not a concept Scout can understand.  She knows
that Mayella is the the one lying, not Helen, and that the Robinson family is kind,
clean, and churchgoing; Scout is also aware that the Ewell family is often cruel or
otherwise "bad," lives in filth and chaos, and exhibits no religious or moral
tendencies.