[Elecraft] W2 Wattmeter 440 band accuracy?

[N2ZDB at aol.com]

I just purchased a new Yaesu digital handheld and the power level on the  
440 band is low (2.8 v 5.0 Watts), and was wondering if I can trust the W2's  
accuracy? Another mobile higher powered transceiver also reads low on all 
power  levels on the 440 band.
 
I built the kit version of the W2 and have the VHF/UHF  coupler....
 
Why is there a +/- calibration button on the PC interface if the unit is  
calibrated properly?
 
I can send my HT back to be tested and exchanged if it isn't making specs  
but don't want to be told that
my wattmeter reading was inaccurate!
 
73's Michael
n2zdb