question about measure dynamic power with HSPICE

Hi,
I want to measure the dynamic power consumption of my VCO in HSPICE.
when I use
.measure tran avgpwr AVG P(v(0)) FROM=10n TO=40n
.measure tran avgpwr AVG POWER FROM=10n TO=40n

Here v(0) is my vdd source.

I get different results. My circuit only contains one VCO.

Could anybody tell me why this happens?

Thank you
teraze