A really basic question about ammeters and their ranges
I looked up the working of an ammeter and i found that an ammeter is just a galvanometer with a resistance in parallel 


We call this the shunt resistance of the ammeter. 

The difference between the high current range and the low current range is that the value of this resistance is lower for the high current range. 

If the circuit you're measuring has a relatively low impedance (less than maybe 20x the shunt resistance) as measured from the points you connected the ammeter, then the choice of range will affect the measured current.

But using a higher range will actually give you a more accurate result, because its lower shunt resistance will affect the measured circuit less.

(On the other hand, the result will be less precise because each step of the ADC in the ammeter will correspond to a larger increment of current)

In comments you added,


  it is a digital meter


In this case, there's no actual galvonometer in your meter. But just the same, the principle is to measure the voltage across a low-valued shunt resistor. And a larger shunt resistor is needed for the lower ammeter range, leading to the same result: The higher range affects the measured circuit less than the lower range.