The secret weapon of stylometry -- the statistical analysis of writing style -- is something called “function words.” Most people assume AI looks for unique topical vocabulary. It actually looks for words like the, and, of, and in. Authors use these filler words unconsciously. Because you do not think about them, they are incredibly difficult to fake or manipulate. You naturally drop them into sentences at a highly specific, mathematical rate, making your "word print" almost wholly unique.

Edit : Since this seems to be an AI summarization (the whole site seems to be, upon closer investigation), here's the original journal article from PLOS .