MSP430FR6989: GPIO static current draw in LPM4

Part Number: MSP430FR6989

I am trying to measure the current draw of MSP430FR6989 in LPM4 on its development board. I measure the current draw from a Keithley SourceMeter.

If I don't unlock GPIOs (don't execute PM5CTL0 &= ~LOCKLPM5;), the current draw in LPM4 is quite close to the number claimed in the datasheet. For example:

However, if I unlock GPIOs, no matter what settings of GPIOs are, the current draw increases with supply voltage in a seemingly exponential way. For example:

I guess the reason might be with CMOS leakage on those GPIO pins but I am not sure of it.

Does anyone know the reason? What does clearing LOCKLPM5 actually take an effect in hardware? 

Thanks in advance. 

Jie Zhan