That doesn't sound normal!

Each individual tooth on the front sprocket comes into contact with a chain roller three times as often as a tooth does on the rear. Theoretically you'd expect the rear to last three times as long as the front. In reality, it's more like two fronts to one rear in my experience.

Either the rear sprocket was poorly manufactured, or was exposed to sand or abrasive particles, or maybe silty water for long periods? Exposure, I'd suggest, is the most likely cause, as the front sprocket is covered and better protected and the rear hangs low and closer to the ground; a problem particularly in deep ruts. Were you riding in ruts, sand, or sandy conditions a lot of the time?

I've never, in all my years of biking, had to change a chain and sprocket set because the rear sprocket wore first. It has always been the chain or front the fails first. Your issue is a weird one!

As for x-ring versus o-ring, they're both good and perfectly up to the task, it's just the x-ring design of seal acts as a wiper as well as a seal and keeps things cleaner.

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