Hey there!
The reasoning behind this is that when you pair your dual-sided power meter with TrainerRoad via ANT+, it measures and communicates L+R power. When paired via Bluetooth, it takes the Left power and doubles it to determine your power output.
Therefore, pairing with ANT+ gives you a more accurate measurement of your total power, and it also will more closely match your outdoor power.
I hope that clears things up!