Like all statistics, these are just summaries of probabilities. You can have two 100-year floods within a few years of one another, by chance. Irregularities in a longitudinal profile are eroded down. Lakes are filled in by deposition. Given enough time, they will all fill.
Valleys become deeper and wider.
Surrounding hillsides are eroded into flatter, smoother forms. These events are the primary shaper of fluvial landforms. Result is that geomorphic work occurs in pulses associated with floods.