I seldom ride with someone else, when I do I ride staggered. I am not a steady enough rider to ride side-by-side. I would guess that is why the one bike is crowding the centerline, the lane is really not wide enough for two bikes running next to each other.

I always ride the left-side 'wheel track' in my lane, gives me a little more room to maneuver if needed. The center of the lane can get slippery from oil droppings and the right side of the lane is 5 feet closer to going off the road.