ADS1278: sample points can't be changed in a fixed sample rate

Part Number: ADS1278

i want to sample a standard 19.2k sine wave by using ads1278 in the following configure.

Because i want to use 38.4k sample rate to sample the 19.2k sine wave, so there should be 2 sample points in every cycle.But the real results don't like what i think.

The real results is listed as the follow figure.You can see that the sample points is not fixed as the same position.But i really don't know why the sample points can't be fixed as i want.

And the code is sample and listed below. The chip is TMS470.

void isr_38k4_timer(){
    spi_send(0x00);
    ad_read[0] = spi_read_buff;
    delay(1us);
    spi_send(0x00);
    ad_read[1] = spi_read_buff;
    delay(1us);
    spi_send(0x00); 
    ad_read[2] = spi_read_buff;

    ad_value = (ad_read[0] << 8) + (ad_read[1] << 8) + ad_read[2];
}

It has really confused my for a week. i want to know that this phenomenon is normal or not. And how could i solve this problem. THX!