Metering Issue: -23dB tone reading as-3LU in Insight

[JasonSoundDesigner]

I noticed my levels weren't matching up so I've been investigating.
I set up a -23dB sine wave with Avid's Signal Gen on a mono audio track. I have Izotope Insight set up on my master fader and it's reading -3LU/-26LUFS (Momentary Max). PT metering set to 'Linear'.

It's my understanding that -23dB is equal to 0LU/-23LUFS and I can't work out where I'm getting the -3dB drop.

I've tried having Insight on the track insert instead of the master and have the same issue.
I have recorded a clip of the -23dB generated tone and imported the file in to Izotope RX and it's giving a similar reading.

What am I missing...?
See screenshots attached