For example, Daedalus has a 25% probability to critical strike for 2.7x damage. On the first attack, however, it will only have an ~8.5% probability to crit; this is its hidden PRD constant. Each subsequent attack without a crit increases this probability by 8.5%. So on the second attack, the chance is 17%, on the third it is 25.5%, etc. After a crit procs, the probability resets to 8.5% for the next attack. These probabilities average out so that, over a moderate period of time, Daedalus will crit very nearly 25% of the time.