Here are some more possible reasons:
You used an Oxford comma.
You didn't use an Oxford comma.
You spelled out a number.
You didn't spell out a number.
You inserted a comma after a short introductory phrase.
You didn't insert a comma after a short introductory phrase.
You wrote "smartphone" instead of "smart phone."
You wrote "smart phone" instead of "smartphone."
You used an en dash instead of an em dash.
You used an em dash instead of an en dash.
You put one space instead of two between sentences.
You put two spaces instead of one between sentences.
The grammar checker in Word put a squiggly line under a poorly structured sentence.
The grammar checker in Word put a squiggly line under a perfectly structured sentence.