twh, it doesn't really work that way. Indeed, the usual math we use is close enough for daily work - but you've hit on one of the areas where things just don't seem to add up. There's a reason for this.
Remember, we're talking AC. The voltage is constantly changing. For the math to add up, you need to use calculus - especially when you have two 'legs' that are not exactly 180 degrees out of phase.
That's also why there sometimes appears to be a difference between VA and watts.
