I have an event on the timeline with stereo audio. The L channel was recorded as a significantly louder level than the R, as indicated by the attached screenshot.

What i don't get is why, when I pan R, it plays back at about the same level as when I pan L, although when Pan centered, R is significantly quieter than L.

Any thoughts?

You can right click the event and split it into its individual channels. Then you can adjust the volume separately and pan them both center.