There are probabilities involved in rare draws since even normal rare pulls are not fixed at 60.

When people give you an estimate it reflects the most common scenario, not every possible scenario. Hell depending how this might be coded you could get a super rare at 1800 pulls (which should be as probable as getting it at only 200 draws).

Expect it once you hit 900. If you get it earlier you were lucky (like the people who got it at 660 draws), if you are unlucky you wil need 1200.

Just pray and/or sacrifice an orphan to RNGesus