When meteorologists talk about a "100-year flood" or a "1,000-year flood," they don't actually mean that the event will happen once every 100 or 1,000 years. Rather, it is a statistical way of expressing the probability of something like this happening in a given year. The chart below is from the USGS (United States Geological Survey), explaining the meaning of these floods.
Within the science of climatology and meteorology, these floods are viewed statistically, as in a 1 in 100 or 1 in 1,000 chance of happening. So in reference to the Ellicott City flood over the weekend, there was only a 1 in 1,000 chance of that happening, or .1 percent. This explains why some Marylanders recall Ellicott City being hit by bad floods before 2016's "1,000-year flood". If anything - it's still impressive that this happened again so soon knowing these odds.
**The National Weather Service has a great report available on this topic for the flood that happened in 2016. The link is listed below.**