It might be more obvious if you increase the volume.
Immagine that your employer has instructed you to sell 1 bottle for $3, but you can sell a whole pallet containing 500 bottles for $1000 (coming out at $2 per bottle). So you ring up the whole pallet and pay it off over the next few days by adding $2 to the cash register and keep $1 for each sale. Over those days, you have made one single sale of a pallet, while pocketing $500.
The work hours which was meant for you to generate sales of $3 bottles has been reduced to effectively selling $2 bottles, while the remaining expected value ended in your pocket.
As others have said, I don’t really care about big corporations losing out on some money, but you are 100% stealing when you reduce the expected sales value and pocket the difference during your work hours.