Help understanding metering levels

I am trying to deeply understand the various meter plugins I have and the Cubase meters. I have an audio test tone sine wave generated from a first class test tone generator website, definitely 1kHZ at -20 dbFS. The Cubase track meter shows input as -20dbFS. But the Stereo Out shows -23dbFS, as does the meter in the right zone. I don’t understand this. There are no inserts anywhere, and just in case the panners attenuate as some do I have disabled the panners.

A 3dB difference sounds to me very much like some sort of standards difference (AES17?), but why should there be a difference in the same mix console?

However, when I use the Steinberg TestGenerator plugin, the stereo out shows -20dbFS when the generator is set to -20dbFS. I can’t quite feel my way around this.

Apologies if this is a dumb question from a relative newbie.

Aha. More internet searching hinted at the Stereo Pan Law setting in the Project setup. In this test case setting it to 0dB addresses the issue. Now to learn about what the Pan Law settings are all about and what they mean.

Without the Pan Law the signal would sound louder when panned center than it would when panned hard left or right. The Pan Law is so the signal level remains constant as you move it around the panorama. As a practical issue this only really matters if the pan position changes over the course of the song. If the panning is static & you set the level by ear you’ll just naturally move the fader a bit higher or lower for different pan laws until the level sounds ‘just right’

Try taking a test tone and sweep it across the panorama using different Pan Laws to hear the results.