A one hundred year flood is not to be expected on a 100 year cycle. It SHOULD mean that in any year there is a one in a hundred chance of such a flood happening.
In practice such floods do sometimes occur more frequently than might be expected, perhaps because of local changes to terrain, changes in land use, or perhaps because the initial observations happened to be made during a long run of unusually benign weather.
Likewise a one thousand year flood SHOULD mean a one in a thousand chance each year, NOT a thousand year rota.
Such events also sometimes seem to occur more often than might be expected, for reasons already given.
Also in most locations there are no accurate records going back that far, so a lot of estimating is involved.
A basic understanding of statistics also explains a lot ! Take a large country like the USA and make a list of the thousand largest towns.
On average, a "once in a thousand year" flood might be expected once in a thousand years AT ANY SPECIFIC LOCATION, or about once a year somewhere.
And remember that some estimates of flood risk may be a simple lie in order to ramp up real estate prices.
"above the thousand year flood line" might a simple advertising claim, especialy if made by someone selling new homes.
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