Think of it this way.
Using very big numbers so it is easy to understand:
Let me say I have a 50% chance of avoiding having an item break every time a tick happens.
If two ticks happen, then using the false addition, I would get the result of 50%+50%=100% chance of avoiding having an item breaking, which is wrong. Perversely, if I then claim that it was a 50% chance of the item breaking, it would be a 100% chance of the item breaking after two ticks, which is false.
Realistically you should multiply each other, so X^T where T is the ticks and X is the % would give me 50%^2 or simply put .5^2 which is .25 or 25% which is correct for the chance of me avoiding a break chance, or to put it in other terms, a 75% chance that an item would break in two ticks.
In your case, the chance of avoiding an item breaking is .96^5 for five ticks.