Sounds to me like maybe one bulb is newer than the other, or they are made by different manufacturers. Try replacing both bulbs at the same time using the same manufacturer and see if that corrects the problem.
Typically it's because the lights are pigtailed together, starting on the driver's side then running through the core support over to the passenger side, so, naturally the driver's side is brighter because it's the first light. There are aftermarket light relay conversion kits that correct this problem to evenly distribute the power to both. I've read this is typical for the headlights, I'm assuming it's the same for the turn signals b/c they're set up the same way (pigtailed).
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