On my bike, the rear harness uses a pair of identical black wires to each indicator. A normal bulb will glow no matter which way the current flows I guess, so it doesn't affect function if you have normal bulbs. The rear bulbs can therefore be wired up either way, it's just random chance how the Triumph tech who assembled it all put in the wires. Guess he got it right on one side and wrong on the other. I had to use the multimeter to measure which one was ground.
The brightness is no doubt just the way the cluster was made. My clusters have an orange center of LED's (24) and a red ring around the circumference (24) that are quite bright as running lights and very bright as brake lights.