MSP430G2553 DCO CLOCK FREQUENCY VARIES WITH FOR LOOP

Hi. I'm trying to blink led with 1second frequency using SMCLK sourcing from DCO. I set clock frequency to 1Mhz. When i set the CCRO=50000 and interrupt counter(led_counter)  to 20 the led blinks wiith a faster frequency(faster than 1 second). If i change CCR0=25000 and interrupt counter to 40 the blinks significantly slower (approx 2 second) than the previous situation. Again with CCR0=12500 and interrupt counter = 80 blinking rate falls nearly 3-4 second.

I couldn't find out where the fault is. Can someone explain this problem ?

include <msp430.h>
volatile int led_counter=0; // Interrupt counter

void main(void)
{
   WDTCTL = WDTPW | WDTHOLD; .
   BCSCTL1 = CALBC1_1MHZ; 
   DCOCTL = CALDCO_1MHZ; 
   P1DIR |= BIT0;                        // Port1.0 output.
   CCTL0 = CCIE; 
   CCR0 = 50000; // 
   TACTL = TASSEL_2 + MC_1; 
   _BIS_SR(LPM0_bits + GIE);
   while(1);
   return 0;
}

#pragma vector=TIMER0_A0_VECTOR 
__interrupt void timer_isr(void)
{
     led_counter++;    
     if(led_counter==20)
     {
         P1OUT ^= BIT0; // Port1.0'ı tersle.
         led_counter = 0; 
     } 
     CCR0 += 50000; 
}