Template:1-1000drop

The table shows the cumulative odds of getting at least one item given a 0.1% drop rate (one in a thousand)

I.E. a 1/1000 drop rate does not mean that if you kill 1000 you WILL get one; rather since the chance is 0.1% each time you kill the mob, then the odds of getting at one from N kills is 100% minus 99.9%N. Furthermore, since the cumulative odds never quite reach 100%, it is possible—albeit unlikely—to never get one.