When you calculate MPG, you're simply dividing Miles driven Per Gallons of gas used. In this simple formula, your gallons is the denominator (bottom of the fraction for those who may have forgotten). Having a larger denominator means each fractional part becomes smaller and smaller.
It all comes down to math and really statistics. The smaller the fractional parts, the less inherent sampling/testing error there is in the calculation. This is to say that the natural sampling and testing error (which can be quite large in MPG calculations), gets broken up more and more as the fractional size gets smaller. For example, you're calculated 36 MPG could be a valid average, but your error may be +/- 20 MPG (I'm just guessing here) since you didn't even use a gallon of gas. Had you had the ability to reduce your units of measure, say meters/milliliter, your short trip would be much more valid (though still likely too small of a sample size to get a statistically accurate result).
I bet if I wanted to, I could "fill up" my T4R, drive 15 miles, refill the tank and be near 0.00 MPG because I "topped off" the tank by having the nozzle click off a few times during refilling.
To get a realistic measurement, fill up until the pump clicks off (no topping off) at the slowest pump speed. Drive at least 3/4 of the tank of gas and refill. Divide the number of miles driven since the last fill up (Trip Odometers work nicely for this) and divide by number of gallons it takes to refill until the pump clicks off in the same manner. This will give you an value with less error than your 36 MPG measurement.
Do this a few times and average the average and you'll likely get a very realistic real-world measurement for expected MPG's.
