Apparently her polished prose was the result of another’s polishing. This is all very unsettling; what next? – Bach was tone deaf? D. H. Lawrence was literate? It’s all too much.
From here:

Yet Jane Austen couldn’t spell, had no grasp of punctuation and her writing betrayed an accent straight out of The Archers, according to an Oxford University academic.

Prof Kathryn Sutherland said analysis of Austen’s handwritten letters and manuscripts reveal that her finished novels owed as much to the intervention of her editor as to the genius of the author.

Page after page was written without paragraphs, including the sparkling dialogue for which Austen is known. The manuscript for Persuasion, the only one of her novels to survive in its unedited form, looks very different from the finished product.

“The reputation of no other English novelist rests so firmly on the issue of style, on the poise and emphasis of sentence and phrase, captured in precisely weighed punctuation. But in reading the manuscripts it quickly becomes clear that this delicate precision is missing.

“This suggests somebody else was heavily involved in the editing process between manuscript and printed book,” Prof Sutherland said.

The editor in question is believed to have been William Gifford, a poet and critic who worked for Austen’s second publisher, John Murray.

“Gifford was a classical scholar known for being quite a pedant. He took Austen’s English and turned it into something different – an almost Johnsonian, formal style,” Prof Sutherland said.

“Austen broke many of the rules for writing ‘good’ English. Her words were jumbled together and there was a level of eccentricity in her spelling – what we would call wrong.

“She has this reputation for clear and elegant English but her writing was actually more interesting than that. She was a more experimental writer than we give her credit for. Her exchanges between characters don’t separate out one speaker from another, but that can heighten the drama of a scene.

“It was closer to the style of Virginia Woolf. She was very much ahead of her time.”

Amongst Austen’s grammatical misdemeanours was an inability to master the ‘i before e’ rule. Her manuscripts are littered with distant ‘veiws’ and characters who ‘recieve’ guests.