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internal RTC works wrong

I have written the following code to set and then read the time. but when I see the result in the LCD, the value of seconds is from 0 to 89 with some jumps like 10 to 16, 25 to 32, 42 to 48 and 73 to 80.

void RTC_Set(void)
{
        RTC_DateTypeDef RTC_DateStructure;
        RTC_TimeTypeDef RTC_TimeStructure;
        RTC_InitTypeDef RTC_InitStructure;
        RTC_AlarmTypeDef RTC_AlarmStructure;
  uint8_t  uwSynchPrediv = 0xFF;
  uint8_t  uwAsynchPrediv = 0x7F;
  /* Enable the PWR clock */
  RCC_APB1PeriphClockCmd(RCC_APB1Periph_PWR, ENABLE);


  /* Allow access to RTC */
        PWR_RTCAccessCmd(ENABLE);

  RCC_LSEConfig(RCC_LSE_ON);


  /* Wait till LSE is ready */
  while(RCC_GetFlagStatus(RCC_FLAG_LSERDY) == RESET)
  {
  }

  /* Select the RTC Clock Source */
  RCC_RTCCLKConfig(RCC_RTCCLKSource_LSE);
  /* ck_spre(1Hz) = RTCCLK(LSE) /(uwAsynchPrediv + 1)*(uwSynchPrediv + 1)*/


  /* Enable the RTC Clock */
  RCC_RTCCLKCmd(ENABLE);


  /* Wait for RTC APB registers synchronisation */
  RTC_WaitForSynchro();

  /* Configure the RTC data register and RTC prescaler */
  RTC_InitStructure.RTC_AsynchPrediv = uwAsynchPrediv;
  RTC_InitStructure.RTC_SynchPrediv = uwSynchPrediv;
  RTC_InitStructure.RTC_HourFormat = RTC_HourFormat_24;
  RTC_Init(&RTC_InitStructure);

  /* Set the alarm 05h:20min:30s */
  RTC_AlarmStructure.RTC_AlarmTime.RTC_H12     = RTC_H12_AM;
  RTC_AlarmStructure.RTC_AlarmTime.RTC_Hours   = 0x05;
  RTC_AlarmStructure.RTC_AlarmTime.RTC_Minutes = 0x20;
  RTC_AlarmStructure.RTC_AlarmTime.RTC_Seconds = 0x05;
  RTC_AlarmStructure.RTC_AlarmDateWeekDay = 0x31;
  RTC_AlarmStructure.RTC_AlarmDateWeekDaySel = RTC_AlarmDateWeekDaySel_Date;
  RTC_AlarmStructure.RTC_AlarmMask = RTC_AlarmMask_DateWeekDay;

  /* Configure the RTC Alarm A register */
  RTC_SetAlarm(RTC_Format_BCD, RTC_Alarm_A, &RTC_AlarmStructure);

  /* Enable RTC Alarm A Interrupt */
  RTC_ITConfig(RTC_IT_ALRA, ENABLE);

  /* Enable the alarm */
  RTC_AlarmCmd(RTC_Alarm_A, ENABLE);

  RTC_ClearFlag(RTC_FLAG_ALRAF);

  RTC_DateStructure.RTC_Year = 15;
  RTC_DateStructure.RTC_Month = RTC_Month_June;
  RTC_DateStructure.RTC_Date = 26;
  RTC_DateStructure.RTC_WeekDay = RTC_Weekday_Friday;
  RTC_SetDate(RTC_Format_BCD, &RTC_DateStructure);

  RTC_TimeStructure.RTC_H12     = RTC_H12_AM;
  RTC_TimeStructure.RTC_Hours   = 5;
  RTC_TimeStructure.RTC_Minutes = 20;
  RTC_TimeStructure.RTC_Seconds = 10;

  RTC_SetTime(RTC_Format_BCD, &RTC_TimeStructure);
}

void RTC_Get(void)
{
RTC_TimeTypeDef RTC_Time_struct;
uint8_t h,m,s;
char str[500];

RTC_GetTime(RTC_Format_BCD, &RTC_Time_struct);
//show in display
h=RTC_Time_struct.RTC_Hours;
m=RTC_Time_struct.RTC_Minutes;
s=RTC_Time_struct.RTC_Seconds;
sprintf(str,"H=%d,M=%d,S=%d",h,m,s);

        GLCD_GoTo(40,4);
        GLCD_WriteString("Time:");
        GLCD_GoTo(40,5);
        GLCD_WriteString(str);
}