Who pays for the extra fuel?

Dear Editor,
Whenever we go to refill fuel at depot, they fill more fuel as compared to what we pay. It has been regularly noticed by everyone.
Mr. X has asked to refill fuel for Rs 500/-, but instead of that they refill for say Rs 501/-, Rs 502/- etc. So, now the question is who bears these extra bucks?
In spite of giving more fuel to us, these depot owners never go in loss. Then, who losses and who make a profit?
We see hue and cries whenever there is any price hike in petroleum products, even if a paisa is hiked but the scene is different in our Capital Complex. Why?
Should we blame the obsolete machines that they use or this is a foul play to dupe people?
Concerned Citizen