system_LPC13xx.c problem

I see this code lines in system_LPC13xx.c for clock update:

LPC_SYSCON->SYSPLLCLKUEN = 0x01; /* Update Clock Source */
LPC_SYSCON->SYSPLLCLKUEN = 0x00; /* Toggle Update Register */
LPC_SYSCON->SYSPLLCLKUEN = 0x01;
while (!(LPC_SYSCON->SYSPLLCLKUEN & 0x01)); /* Wait Until Updated */

but it seems that lines 1 and 4 in above code is useless because in datasheet UM10375.pdf we see:

In order for the update to take effect, first write a zero to the SYSPLLUEN register and then write a one to SYSPLLUEN.

why we first one in code?
why we check ENA bit going low?there is not note about this in datasheet.(line4)

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In line 352 ( system_LPC13xx.c ) we have:

if (LPC_SYSCON->SYSPLLCTRL & 0x180) {

but in SYSPLLCTRL register bit 0~6 are used and others are reserved,why in above bits 8 and 7 are used in if statement?