Why the wrong frequency HSI STM32F103VET6?

[VOvs.1]

I have MCU STM32F103VET6, the microcontroller is clocked from HSI (internal 8MHz oscillator).

Bun the frequency at the MCO pin is 6.4MHz. Why? 

I have the same MCU bought years ago, with the same code the frequency is correct - 8MHz. 

Here is my code:

void SystemInit (void)
{
  // Set HSION bit */
  RCC->CR |= (uint32_t)0x00000001;
  // Reset SW, HPRE, PPRE1, PPRE2, ADCPRE and MCO bits
  RCC->CFGR &= (uint32_t)0xF8FF0000;  
  // Reset HSEON, CSSON and PLLON bits
  RCC->CR &= (uint32_t)0xFEF6FFFF;
  // Reset HSEBYP bit
  RCC->CR &= (uint32_t)0xFFFBFFFF;
  // Reset PLLSRC, PLLXTPRE, PLLMUL and USBPRE/OTGFSPRE bits
  RCC->CFGR &= (uint32_t)0xFF80FFFF;
  // Disable all interrupts and clear pending bits  
  RCC->CIR = 0x009F0000;
 
  SCB->VTOR = FLASH_BASE | VECT_TAB_OFFSET; 
}

[Uwe Bonnes]

Where do you enable MCO?

[VOvs.1]

void main () { 
  GPIO_InitTypeDef  GPIO_InitStructure;
 
  RCC_APB2PeriphClockCmd(RCC_APB2Periph_GPIOA, ENABLE);
 
  GPIO_InitStructure.GPIO_Pin = GPIO_Pin_8; 
  GPIO_InitStructure.GPIO_Mode = GPIO_Mode_AF_PP;
  GPIO_InitStructure.GPIO_Speed = GPIO_Speed_50MHz;
  GPIO_Init(GPIOA, &GPIO_InitStructure);
 
  while(1) ;
}

[waclawek.jan]

In these days, when it comes to 'F103, the first question is, where do you have them from/aren't they counterfeit (I know it's more likely the 'F103C8, but still).

What happens if you replace the whole SystemInit() by an empty function?

What's the value of RCC->CR, namely the HSICAL/HSITRIM fields?

JW