MC at 8-12 weeks

How common are miscarriages between 8 and 12 weeks? Everything I’ve read says that the risk goes down to 4-5% after a heartbeat is confirmed at 8 weeks. But yet, I’ve seen so many posts about a great heartbeat and yet a miscarriage at 10 or 11 weeks :(. How is that there seem to be so many and yet the risk is so low? Makes me so nervous.