Flip a coin twice. Yes they are independent events and both have a 50% to come out as heads but since their are 4 possible outcomes of happening (tails-tails,tails-heads,heads-tails,heads-heads) that makes it 1/4 chance of both being heads. Same rule applies to xur if every item has a equal chance. Next week Friday there is a 1/6 chance he will have void bang vestment but for him to have it 5 times in a row that chance is extremely unlikely but it's already happened 4 times so we will see