Why does Grammarly (or spell-check and editing) make my essay score as AI?

Same thing happens to me and I write for a living. Clean copy scores higher. Short sentences, no filler, every comma in the right place. That is what editing produces. It is also what a language model produces. So the detector sees "polished" and reads "machine".

Grammarly did not put a mark in your file. There is nothing in a .docx that says "Grammarly was here". The detector only ever sees the words.

Keep your drafts. Keep them with timestamps. That has saved me twice with clients.

Aisha, for what it is worth, I see this every semester. The students who get flagged are often the careful ones, the ones who proofread. The students who dash something off at midnight with run-on sentences almost never trip it.

One question before anyone gives you advice: which Grammarly features are you actually using? The underline corrections (spelling, grammar, punctuation) are one thing. The "improve it", "make it sound more academic", "rephrase" buttons are a different thing entirely. Those rewrite whole sentences with a language model, and that output does look like AI because it is AI. If you only accept the underline fixes, you are on much safer ground than you think.

Minor technical point since it keeps coming up: Turnitin can tell you used Grammarly. The browser extension and the Word add-in both touch the document, and the detector fingerprints that. It is not just the prose. So the honest advice is to write in a plain editor with no extensions if you want a clean score.

@Sam Whitlock I do not think that is right, and I would rather we did not spread it. Turnitin's detector takes the submitted text, splits it into overlapping segments and scores each one; that is how Turnitin describes it in the FAQ Maya linked. It has no access to which tools edited the file, and a .docx carries no record of a Grammarly session. The score rises because the sentences became more uniform, not because of a fingerprint. Writing in a plain editor would change nothing if the resulting text is the same.

@Maya Lindqvist thank you, this is a relief. I only use the underline corrections, never the rewrite button. I did not know about the Authorship feature. I will turn it on and also keep my early drafts in a folder with the dates. I feel less afraid of submitting my next essay.

As a TA I want to add the practical side. When a score comes across my desk, the first thing I ask for is drafts and the second is a short conversation about the essay. A student who can walk me through why they ordered the argument the way they did has nothing to worry about from me, whatever the percentage says.

I tested this once myself: I took three paragraphs from my own thesis, ran them through a checker, accepted every Grammarly suggestion, and ran them again. The score moved up. Same ideas, same structure, tidier sentences. That was enough to convince me that a number on its own tells me very little, and I say so to the instructors I work with.