The 100 year return or recurrence frequency doesn't mean it's only going to happen once in 100 years or that it's a 1% chance collectively over 100 years. The return/recurrence water levels is what the 1% is based on but it's terminology more common and familiar to those dealing with singular rainstorm events, at least before those regulating floodplains tried to do a better job of explaining it. The storm recurrence is just one of the hydrologic variables that go into overall hydraulic (the physics part) modeling that comes up with the flood elevations used for floodplain mapping of designated zone like the Zone A (1%).
USGS has a nice summary of it all in the this linked pdf:
https://pubs.usgs.gov/gip/106/pdf/100-year-flood-handout-042610.pdf
"In the 1960’s, the United States government decided to use the 1-percent annual exceedance probability (AEP) flood as the basis for the National Flood Insurance Program. The 1-percent AEP flood was thought to be a fair balance between protecting the
public and overly stringent regulation. Because the 1-percent AEP flood has a 1 in 100 chance of being equaled or exceeded in any 1 year, and it has an average recurrence interval of 100 years, it often is referred to as the “100-year flood”. "
"The 1-percent AEP flood has a 1-percent chance of occurring in any given year; however, during the span of a 30-year mortgage, a home in the 1-percent AEP (100-year) floodplain has a 26-percent chance of being flooded at least once during those 30 years! The value of 26 percent is based on probability theory that accounts for each of the 30 years having a 1-percent chance of flooding."
If you want to be optimistic you can always go with "Hey, there's a 74% chance over 30-years I won't be flooded!" Your mortgage company might disagree in you are in Zone A though.
And just to make it more confusing every damn high water event and flood is call a "100-year flood" by locals. The term just can't die! Most are not to that level. You can have a major flood, especially for low lying river towns, that causes damage and inconvenience and they are well below the 1% regulatory elevation established for the community.