It's kind of an abuse of the term. The technical definition is a storm that according to historical records has a 1% chance of happening in any given year. So you'd expect to see it at least once in 100 years. As climate change increases the severity of all types of weather those level of storms will get more common than in the past in a hard to predict way. The storm percentages are also based on the historical record instead of future predictions so it will inherently lag behind the expected increasing storm frequency/intensity.