Since horus brought it up, I'd like to suggest there to be some random factor to the stats on the trees. Of course the base amount still depends on the tree's age, but currently, every single petrified oak tree gives exactly 97 speed.
I think there could be some variation, like the balances on weapons. And the older the tree, the bigger the variation, so you can get as low as 80 speed on the above staff, but also as high as 110 (like the helpfile suggests)