So what's the math/logic behind the percieved discrepencies between % and actual rate

[Zykor]

This is where we get into probability. Let's say that we flip a coin 10 times, each next event is of course a 50% chance of getting heads. However if you want to calculate the chance of getting heads 10 times in a row, you would have to take 1/2 times itself by 1/2 ten times, or 0.5*0.5*0.5...and so on.

The end result would be 1/1024, or 0.00097 (0.097%, almost a tenth of a percent).

So the chance of you missing 5 times in a row at 95% success rate item?
Take the 5% failure rate and multiple it by five times, which looks like:
.05*.05*.05*.05*.05 = 0.0000003125

Therefore, the chance of you failing a 95% success rate item five times in a row can be viewed as that sucks mate.

EDIT: For your 33% proc chance, failing at 15 times, would be .67*.67 up to 15 times, which results in a 0.00246, or 0.246%