How do you figure that? If you do 60 runs, you will get 10 boots, if you do 6000, you will get 1000 boots
By saying the average number is 3.5 you're implying you're about twice as likely to get boots as any other piece.
The numbers i gave were reduced to the average number of runs you need to do to get anything in particular, over a large set of runs.
When you talk about one particular instance possible amongst thousands, you cannot simplify it as "well, one player will get it on the first try". Maybe that's so, but one player will run it 100 thousand times and still not get them. All those numbers above were averages. Not total numbers for guaranteed results. But it gives you an idea. If you dont run these kinds of totals, you cannot reasonably expect to get the best items.
If you do 6 runs we would expect to get one each weapon, helm, etc. Half of those would come by the 3rd run, and half would come in the 4th through 6th run. It's no different than rolling a die. Half the numbers would be expected to come up by the 3rd roll, and half after the 3rd roll. We wold expect, in six rolls, to roll once each a 1, 2, 3, 4, 5, and 6? That adds to 21. The average per roll is 21/6 = 3.5.
What you are saying would be equivalent to saying if 6 people rolled a die, only 3 of them would roll a six by the sixth roll, and 3 of them would still be looking for a six after the sixth roll.
You are correct that if you do 60 runs you will get 10 boots. But you will get half of those boots by the 30th roll. If you roll a die 6 times, you will get 1 six. Half the time by the 3rd roll, half the time after. If you do six runs, you will get one pair of boots. Half the time by the 3rd run, half the time after.
If 6 players do 6 runs, one will get boots on the first run, one will get them on the second, one will....get them on the sixth run. The average number of runs to get boots = 3.5. Same for weapons etc.