Probability. If something has a 50% chance of occuring, that does not mean it will happen every second time, and our brain has a very hard time rationalizing that. For example, we assume its near impossible to flip heads on a coin three times in a row when really, the probability is 12.5% - not that low. Another example would be something with a 95% chance of success - we naturally round up and assume thats basically garenteed success, but theres still a very decent chance of failure, esspecially on repeat attempts. Our brains are just not wired to handle randomness well, which is part of why gambling is so addicting and why games like X-Com have to rig the odds in the players favour to avoid pissing them off.