If you play several consecutive levels without closing the game, the stat of 'picked up floor money' will continue to increase.
For example if you pick up a total of $4 the first run it will add that to the completed level money properly but if you then have another run and get another $3, the final screen will say +$7 instead.
You can see how after 5+ levels it can be quite a difference in what it says you earned.