The Dry Boiling Point of brake fluid refers to the boiling temperature of fresh, new brake fluid from an unopened container. Whereas the Wet Boiling Point is defined as the temperature DOT brake fluid will begin to boil after it has absorbed 3.7% water by volume. DOT brake fluid will reach this level of water volume after roughly 2 years of service, which is why it is advisable to renew your brake fluid every 1-2 years.
The effects of this water content over time are better illustrated by the graph below. This graph is taken from Shell and gives an overview of the declining effect water content has on its range of DOT 3 and DOT 4 brake fluids.
Let's remember that the Department of Transportation specify the minimum wet boiling point of DOT brake fluids after absorbing only 3.7% water content (roughly 2 years service). In the graph above we can see that the boiling points of the various Shell DOT fluids decline much further over longer periods of time. When brake fluid reaches 8% water content the boiling point of Shell DOT 3 brake fluid has been reduced almost to that of water - 100°C!