TL;DNR: As others have said, the usual formula for max HR is wrong, and it only gives an average, which is lower than roughly 1/2 the population.
Some details:
1) There are plenty of studies showing that the standard formula for maximum heart rate is not accurate. Moreover, the standard formula gets worse with age; as people get older, max HR does not fall as fast as the formula predicts. This means the older you get, the more likely your max HR is above the predicted max HR
2) More importantly, any formula only gives the average max heart rate. Actual max HRs are spread out around that average in a bell curve. The variation (measured by the standard deviation) is quite large. Because the spread is pretty close to a bell curve, ½ of people have max HR above what the formula predicts.
PS A quick search for something like maximum+heart+rate+formula+study will tell you more than you want to know.