You’re measuring at two different points and at those points you have a different amount of channels, 1 vs 2. When you go from a mono track into a stereo path the DAW will have to decide how that should work. If it just copies the mono channel into both the left and right channels in the stereo path then the resulting signal in total would be much louder. This is why you have the option in the project setup dialog to adjust that.
So if you open the setup dialog for the project you’ll see “Pan Law” with a drop-down box with options for how to adjust this. It is basically how much a signal is attenuated to compensate for what I described.
There’s a section in the manual about “Pan Law” so I suggest you pull that up and then just switch that parameter back and forth between different options while you look at the meters. I think that’s easier than me trying to explain it ('cause I’m not good at explaining this).
But that is what you’re looking at I think.