usb host shield + wireless mouse (Read 5 times)

Question

hello guys i have a problem ,

i have a usb host shield v2.0 from circuits @home attached to an arduino mega and to a usb wireless reciever mouse , this is the serial monitor output :

x=0 dy=3
dx=0 dy=-1
dx=0 dy=-1
dx=0 dy=-1
dx=0 dy=-1
dx=0 dy=-1
dx=0 dy=1
dx=0 dy=-2
dx=0 dy=-2
dx=0 dy=2
dx=0 dy=9
dx=0 dy=6
dx=0 dy=-1
dx=0 dy=2
dx=0 dy=-1
dx=0 dy=-1
dx=1 dy=0
dx=2 dy=0
dx=1 dy=0
dx=2 dy=0

and this is the code from the library usbhidbootmouse :

#include <hidboot.h>
#include <usbhub.h>
// Satisfy the IDE, which needs to see the include statment in the ino too.
#ifdef dobogusinclude
#include <spi4teensy3.h>
#endif
#include <SPI.h>
class MouseRptParser : public MouseReportParser
{
protected:
    void OnMouseMove    (MOUSEINFO mi);
    void OnLeftButtonUp (MOUSEINFO mi);
    void OnLeftButtonDown   (MOUSEINFO mi);
    void OnRightButtonUp    (MOUSEINFO mi);
    void OnRightButtonDown  (MOUSEINFO mi);
    void OnMiddleButtonUp   (MOUSEINFO mi);
    void OnMiddleButtonDown (MOUSEINFO mi);
};
void MouseRptParser::OnMouseMove(MOUSEINFO mi)
{
    Serial.print("dx=");
    Serial.print(mi->dX, DEC);
    Serial.print(" dy=");
    Serial.println(mi->dY, DEC);
};
void MouseRptParser::OnLeftButtonUp (MOUSEINFO mi)
{
    Serial.println("L Butt Up");
};
void MouseRptParser::OnLeftButtonDown   (MOUSEINFO mi)
{
    Serial.println("L Butt Dn");
};
void MouseRptParser::OnRightButtonUp    (MOUSEINFO mi)
{
    Serial.println("R Butt Up");
};
void MouseRptParser::OnRightButtonDown  (MOUSEINFO mi)
{
    Serial.println("R Butt Dn");
};
void MouseRptParser::OnMiddleButtonUp   (MOUSEINFO mi)
{
    Serial.println("M Butt Up");
};
void MouseRptParser::OnMiddleButtonDown (MOUSEINFO mi)
{
    Serial.println("M Butt Dn");
};
USB     Usb;
USBHub     Hub(&Usb);
HIDBoot<USB_HID_PROTOCOL_MOUSE>    HidMouse(&Usb);
MouseRptParser                               Prs;
void setup()
{
    Serial.begin( 115200 );
#if !defined(MIPSEL)
    while (!Serial); // Wait for serial port to connect - used on Leonardo, Teensy and other boards with built-in USB CDC serial connection
#endif
    Serial.println("Start");
if (Usb.Init() == -1)
    Serial.println(&quot;OSC did not start.&quot;);

delay( 200 );

HidMouse.SetReportParser(0, &amp;Prs);

}
void loop()
{
  Usb.Task();
}

i wanna know the dx is equal to 1 when i press the left button on mouse and is equal to 2 when i press the right button on mouse and the dy is responsible for the x and y coordinates from - to + ? how a single value is responsible for x and y both ?