LP87702-Q1: I can't change LP87702 Frequency

Hi all:

        From the data sheet, LP87702 supports 2-MHz, 3-MHz, or 4-MHz Switching Frequency,so I find the register FREQ_SEL Register (Offset = 31h).

        But I can't change the value.I write the value for 0x05 or 0x04,the value read from the register is always 0x06.It is mean the 4-MHz.

        So what's wrong?Please give me a answer.