Likely they don't calculate how much they are making after expenses or don't understand how expenses work.

For example, a reddit post I read a little while ago (can't find it again) the person said they do ride sharing so got to take a lot of deductions on their taxes so they said "on paper" it "looks like" they make "much less" than they "actually" do.

The comments rightly pointed out the poster's "on paper" income is their actual income, because they are deducting expenses they paid and it wasn't an accounting trick.