That's not the only contrast here. You may notice that Mrs. Spencer calls Marilla "Miss Cuthbert." And yet in Chapter 7, Marilla's going to say that everyone except the minister calls her by her first name. We're not in Avonlea anymore.
They've gone to Mrs. Spencer's so Marilla can deposit Anne and let the people who can't seem to give each other messages correctly sort it out. Except that Mrs. Spencer's solution is to send Anne off with Mrs. Peter Blewett, which Marilla knows would be a Very Bad Idea.
She knew Mrs. Peter Blewett only by sight as a small, shrewish-faced woman without an ounce of superfluous flesh on her bones. But she had heard of her. "A terrible worker and driver," Mrs. Peter was said to be; and discharged servant girls told fearsome tales of her temper and stinginess, and her family of pert, quarrelsome children. Marilla felt a qualm of conscience at the thought of handing Anne over to her tender mercies.(That last line, by the way? A reference to Proverbs. Thank you, Annotated Anne.)
So in the end, it's the prospect of another "good woman" being responsible for Anne that drives Marilla to claim her. You can't accuse her of taking up the responsibility with false ideals:
"I've never brought up a child, especially a girl, and I dare say I'll make a terrible mess of it. But I'll do my best."
Matthew, unsurprisingly, is delighted. Marilla has just one rule for him:
"Perhaps an old maid doesn't know much about bringing up a child, but I guess she knows more than an old bachelor. So you just leave me to manage her."He has one too:
"Only be as good and kind to her as you can without spoiling her. I kind of think she's one of the sort you can do anything with if you only get her to love you."(Post pic: A house, though it's not yellow. Look closely -- or click for the full-size version -- and you'll see that the front yard features a birdhouse that is a miniature version of the house itself. This is what happens when builders have time on their hands.)