Interfacing Nintendo DS resistive touch screen

Hi,

I'm using arduino leonardo to interface the Nintendo ds touch screen (https://www.sparkfun.com/products/8977)
I used this example as starting point:
http://kalshagar.wikispaces.com/Arduino+and+a+Nintendo+DS+touch+screen

I connected the touch screen in the fallowing way:
Y1 - A2
X2 - A1
Y2 - A0
X1 - A3

I use this two functions to read X and Y:

int readX() // returns the value of the touch screen's X-axis
{
  int xr=0;
  
  pinMode(A0, INPUT);   // A0
  pinMode(A1, OUTPUT);    // A1
  pinMode(A2, INPUT);   // A2
  pinMode(A3, OUTPUT);   // A3
  digitalWrite(A1, LOW); // set A1 to GND
  digitalWrite(A3, HIGH);  // set A3 as 5V
  delay(5); // short delay is required to give the analog pins time to adjust to their new roles
  xr=analogRead(A2);
  
  return xr;
}
 
int readY() // returns the value of the touch screen's Y-axis
{
  int yr=0;
  pinMode(A0, OUTPUT);   // A0
  pinMode(A1, INPUT);    // A1
  pinMode(A2, OUTPUT);   // A2
  pinMode(A3, INPUT);   // A3
  digitalWrite(A0, LOW); // set A0 to GND
  digitalWrite(A2, HIGH);  // set A2 as 5V
  delay(5); // short delay is required to give the analog pins time to adjust to their new roles
  yr=analogRead(A1);
  return yr;
}

The problem is that I get this intervals when readin X and Y:
y= [150,740]
x=[100,470]

Why is the interval on x smaller the on Y axis if the resistance on X axis is 624 and on Y axis is 300 and something ?