Two different things travel under the same word, and confusing them causes most of the pointless edits. Technical originality is the share of your text that no existing page already carries. Editorial originality is whether the page holds anything the other results do not: your experience, your examples, your angle.
Checkers measure the first one. In simple terms they cut the text into short chains of words and look for those chains in their index. Two things follow. The same text scores differently in different tools, and a match is not always borrowing — a standard clause matches every time.
A checker knows nothing about authorship. It sees matching strings, so your own text, published last month on another site, counts as duplicate content just like a stranger’s. Look at what a fragment actually matched before you start editing it. Often it turns out there is nothing to fix.
Aiming for a perfectly original text is usually harmful. Copy stripped of every recognisable phrase tends to lose its terms and read worse. The sensible goal is different: remove the matches that came from laziness, and keep the ones that hold the meaning together.
The practical part is this: work from the highlighted fragments, not from the headline number. That number moves from tool to tool, while the list of matches is a concrete edit plan. Three things usually sit in it — a template opening, definitions copied from a reference, and lists taken almost word for word.