If you would be so kind as to take screenies of your caches and posting them here we might glean some knowledge.
If, for instance, there are mechs in most of the caches, then we will at least know that the odds of getting a mech is not 1 in 8, it's most likely much lower odds than that...say 1 in 500.
I don't have any, not having played yet, but my friends migh get some tonight while we are