Weird.
I've had a defect rate of less than 1% percent with a sample size of enough items to reduce the margin of error on that statistic to something rather negligible.
To get your configuration you need to have two items that aren't defective and five items that are.
P(!defect)^2P(defect)^5 = 0.99^20.01^5 = 9.801e-11
There are seven choose five ways you could have a particular configuration where you have five defects out of seven purchases.
7 Choose 5 = 21
So therefore the probability of what happened to you happening to me would seem to be:
9.801e-11 * 21 = 2.05821e-9
This written out in numbers is: 0.00000000205821 That number as a percentage is: 00.000000205821% For comparison the probability of being struck by lighting over the course of a person's life is 1-in-15,300 which is 0.00006535947. Many people struggle with reading really small numbers like that and so we often decide to round. If we do that the probability of what happened to you happening to me is roughly 0% if we choose to round at six decimal places. Seconding the OP claim that the general experience is shocking, because it is extremely divergent. It would be more shocking to be hearing everyone on Hacker News was regularly being hit by lightning bolts than that people are getting your defect rate.