What you are missing is the fact that you think it isn't random when going the same thing over and over. The problem is you are forcing something. That thing is only going to work if your opponents aren't also trying to use it. If you are the only one going for it then it will work out great. If there are 2 other people trying to get the same champions as you, you are all going to lose miserably while all sitting on one star units until it is too late. IT IS 100% RANDOM so you have to look for what your best odds are. If you see 3 people started with a 2 star Vlad, don't start buying ocean units because you won't get them. The opposite is also true though. If you see someone is doing well with glacial and you are sitting pretty well yourself with resources to spare, pick up every Olaf you see. The same can be said of any comp that depends on certain units to carry. If you see first place is sitting on 3 blademasters, buy up every Yi that you come across.
It is just simple math and it straight up tells you the odds now. If you have a 15% chance to get one of the 5 cost units, that is a 15% chance that one of them shows up, not that any specific one shows up. If someone else already has 5 Nami's, you have a higher chance of getting a Taric because Nami is less likely to show up.
Let's say there are 9 of each 5 cost unit in the shared pool. (Amumu, Zed, Yi, Singed, Taric, Nami)
2 people have 3 Amumus, and one person have 3 of Zed, Yi, Singed, and Taric. This is under the assumption that most people only go for a 2 star 5 cost due to the difficulty of getting a 3 star.
If you have a 15% chance to role a 5 cost, that means you have a 15% chance to role 3 Amumus, 6 of Zed, Yi, Singed, and Taric, and 9 of Nami.
Break that down further and you have a 2.8% chance to get Amumu, 5.6% chance to get one of the 4, and 8.4% chance to get Nami. Point being that while it is up to chance if you get something, there is no point trying to get one thing when you have a higher chance to get something else.
