Configured sampling time does not match with real sampling time

/* Private typedef -----------------------------------------------------------*/

# define N_samples      100



/* Private define ------------------------------------------------------------*/
/* Private macro -------------------------------------------------------------*/
/* Private variables ---------------------------------------------------------*/









ADC_InitTypeDef         ADC_InitStructure;
ADC_CommonInitTypeDef   ADC_CommonInitStructure;
GPIO_InitTypeDef GPIOADC_InitStructure;
/* Private function prototypes -----------------------------------------------*/

        uint32_t index=0;
        float32_t array2[N_samples];


/* Private functions ---------------------------------------------------------*/

/**
  * @brief  Main program
  * @param  None
  * @retval None
  */
int main(void)
{







                //1.  Enable the ADC interface
RCC_APB2PeriphClockCmd(RCC_APB2Periph_ADC3, ENABLE);





//3. Configure the ADC Prescaler, conversion resolution and data
  //  alignment using the ADC_Init() function.

        /* ADC Common Init(Configure the ADC Prescaler) **********************************************************/
   ADC_CommonInitStructure.ADC_Mode = ADC_Mode_Independent;
   ADC_CommonInitStructure.ADC_Prescaler = ADC_Prescaler_Div2; //ADCclock= PCLK2/2= 36Mhz/2=18MHz
   ADC_CommonInitStructure.ADC_DMAAccessMode = ADC_DMAAccessMode_Disabled;
   ADC_CommonInitStructure.ADC_TwoSamplingDelay = ADC_TwoSamplingDelay_5Cycles;
   ADC_CommonInit(&ADC_CommonInitStructure);


        //conversion resolution and data alignment
ADC_InitStructure.ADC_Resolution        =       ADC_Resolution_12b;
ADC_InitStructure.ADC_ScanConvMode      =       DISABLE;
ADC_InitStructure.ADC_ContinuousConvMode        =       DISABLE;
ADC_InitStructure.ADC_ExternalTrigConvEdge      =       ADC_ExternalTrigConvEdge_None;
//ADC_InitStructure.ADC_ExternalTrigConv        =       ADC_ExternalTrigConv_T1_CC1;
ADC_InitStructure.ADC_DataAlign =       ADC_DataAlign_Right ;
ADC_InitStructure.ADC_NbrOfConversion   = 1;

ADC_Init(ADC3, &ADC_InitStructure);

ADC_RegularChannelConfig(ADC3, ADC_Channel_4, 1, ADC_SampleTime_3Cycles);//sampletime= ADC_clock/(3+12)=  18MHz/15=1.2MHz

//2. ADC pins configuration
RCC_AHB1PeriphClockCmd(RCC_AHB1Periph_GPIOF, ENABLE);

GPIOADC_InitStructure.GPIO_Pin = GPIO_Pin_6;//PF9
GPIOADC_InitStructure.GPIO_Mode = GPIO_Mode_AN ;// PIN 9 IN ANALOGUE MODE
GPIOADC_InitStructure.GPIO_PuPd = GPIO_PuPd_NOPULL;
GPIO_Init(GPIOF, &GPIOADC_InitStructure);


//4. Activate the ADC peripheral using ADC_Cmd() function.
ADC_Cmd(ADC3, ENABLE);


                /* Get the N ADC samples */
for (index=0; index<N_samples; index++)
{
/* ADC start conv */
ADC_SoftwareStartConv(ADC3);

/* Wait end of conversion */
while(ADC_GetFlagStatus(ADC3,ADC_FLAG_EOC) == RESET);
/* Store ADC samples*/


array2[index]= ADC_GetConversionValue(ADC3)-2048;