程序测量功率计的“功率参考”输出电平不理解

我在计量实验室工作，并在各种功率计上进行校准。
当我使用大多数服务手册中提到的程序测量功率计的“功率参考”输出电平时，我有点担心我看到的一些含糊之处。
按照程序，我使用的是432B型功率计和478A型热敏电阻安装座，均经过工厂校准和认证，以及经过校准的34401A数字万用表。
我将这个测量结果与我使用新出厂校准和认证的E4419B以及几个经过工厂校准和认证的8480系列功率传感器进行比较。
我刚刚在E4418B功率计上执行了这些测量，并使用手册中调出的方法计算出它为0.981 mW，这意味着我需要调整功率参考以满足规范。
但是，当我测量功率参考时。
使用E4419B和功率传感器，我的读数为1.001 mW，几乎是完美的。
这是我校准的所有功率计的结果。
为什么方法之间的差距是我使用的替代方法？



     以下为原文

  I work in a Metrology Lab and perform calibrations on a wide range of power meters. I am a little concerned with some ambiguities I am seeing when I am measuring the "Power Reference" output levels of power meters utilizing the procedure called out in most of the service manuals. Per the procedure I am using a model 432B Power Meter and a model 478A Thermistor Mount, both freshly factory calibrated and certified and a calibrated 34401A DMM. I am comparing this measurement against one I make using a freshly factory calibrated and certified E4419B and several factory calibrated and certified 8480 series power sensors. I just performed these measurements on an E4418B power meter and using the method called out in the manual calculated it to be 0.981 mW which means I would need to adjust the power reference to meet spec. However, when I measure the power ref. with the E4419B and power sensor I get a reading of 1.001 mW whic is almost perfect. This is quite typical of my results on all the power meters I calibrate. Why the desparity between methods and is the alternate method I use acceptable?