Its like this: for example, your tanks fills up in 300 rupees.
And goes 100 kilometers.
now you fill up 3 times 100 rupees each time. and your car will run about 70 to 80 kilometers only. What happens is that you are building pressure three times.. hence the wasting the gas 3x times.
Now when you fill up your tank, the pressure is build once and is constant (almost) till your tank fills up and hence less gas wastage.. and you get 100 kilometers out of it..
thats what I think.