24bit SysTick Timer is not fixed to half the frequency of the main system clock. As it mention on the PDF Manual on page 210 of chapter 15.2
Please fix the PDF manual.

I have set the IRC clock as a main input clock at 12MHz and this was affect the SysTick Timer Clock which is set to the same clock as the main clock, and not to the half clock as the manual say which is fixed. It is not fixed!

LPC_SYSCON->MAINCLKSEL = 0x00;
LPC_SYSCON->MAINCLKUEN = 0x00;
LPC_SYSCON->MAINCLKUEN = 0x01;
while (!(LPC_SYSCON->MAINCLKUEN & 0x01));
LPC_SYSCON->SYSAHBCLKDIV = 0x01;