Yes its the same chance every week but it compounds. Say its the chance of getting the item is 1/2. Every week the chance of getting it is 1/2. However the chance of getting it 3 times in a row changes. The chance of getting it 3 times in a row is 12.5% i believe, my math may be wrong. The point is that its two different equations. The chance of getting the item will always be 1/2. However the 12.5% decreases every time you get the item because of probability. Go on the terraroa forums and look at "luckiest drops" or something like that. Or any rng thread there.