Why does the CO2 results display results in negatives?

/************************Hardware Related Macros************************************/
#define         MG_PIN                       (0)     //define which analog input channel you are going to use
#define         BOOL_PIN                     (1)
#define         DC_GAIN                      (8.5)   //define the DC gain of amplifier
 
 
/***********************Software Related Macros************************************/
#define         READ_SAMPLE_INTERVAL         (50)    //define how many samples you are going to take in normal operation
#define         READ_SAMPLE_TIMES            (5)     //define the time interval(in milisecond) between each samples in 
                                                     //normal operation
 
/**********************Application Related Macros**********************************/
//These two values differ from sensor to sensor. user should derermine this value.
#define         ZERO_POINT_VOLTAGE           (0.324) //define the output of the sensor in volts when the concentration of CO2 is 400PPM
#define         REACTION_VOLTGAE             (0.020) //define the voltage drop of the sensor when move the sensor from air into 1000ppm CO2
 
/*****************************Globals***********************************************/
float           CO2Curve[3]  =  {2.602,ZERO_POINT_VOLTAGE,(REACTION_VOLTGAE/(2.602-3))};   
                                                     //two points are taken from the curve. 
                                                     //with these two points, a line is formed which is
                                                     //"approximately equivalent" to the original curve.
                                                     //data format:{ x, y, slope}; point1: (lg400, 0.324), point2: (lg4000, 0.280) 
                                                     //slope = ( reaction voltage ) / (log400 –log1000) 
 
 
void setup()
{
    Serial.begin(9600);                              //UART setup, baudrate = 9600bps
    pinMode(BOOL_PIN, INPUT);                        //set pin to input
    digitalWrite(BOOL_PIN, HIGH);                    //turn on pullup resistors
 
   Serial.print("MG-811 Demostration\n");                
}
 
void loop()
{
    int percentage;
    float volts;
     
    
    volts = MGRead(MG_PIN);
    Serial.print( "SEN0159:" );
    Serial.print(volts); 
    Serial.print( "V           " );
     
    percentage = MGGetPercentage(volts,CO2Curve);
    Serial.print("CO2:");
    if (percentage == -1) {
        Serial.print( "<400" );
    } else {
        Serial.print(percentage);
    }
    Serial.print( "ppm" );  
    Serial.print( "       Time point:" );
    Serial.print(millis());
    Serial.print("\n");
     
    if (digitalRead(BOOL_PIN) ){
        Serial.print( "=====BOOL is HIGH======" );
    } else {
        Serial.print( "=====BOOL is LOW======" );
    }
       
    Serial.print("\n");
     
    delay(200);
}