density differences in boxes

I've bought boxes from lots of vendors, and I notice lots of variability in the density of the wood. I'm assessing this based upon perceived weight differences, and hardness of the wood. Seems like Mann lake sells the heaviest and hardest wood that I've seen. Has anyone else observed this? I suspect, but have no real data to support, that there is a direct correlation between density of the wood and life expectancy. Any thoughts?