In your test, did you measure the AH capacity in the discharge or the recharge? The latter is of course generally an overestimate.

The Yuasa had to be really well made to last THAT long! Half the life at half the cost may be OK too though. 6 years is not bad.

The practical life of an SLA mostly depends on how it is treated, not the manufacturing quality. I've seen our club's batteries go dead in less than 2 years. I think that's in part due to sometimes being left in a glider that it tied out, where it gets quite hot. I left a spare battery in my glider trailer for one season, it was then dead, again perhaps due to heat. Other than keeping them cool (but not cold), make sure to fully charge right after use (within a day or two anyway), using a good "smart" charger that will not overcharge it.