The annual amount was calculated by using the stated expiration date, the stated start date, and the stated award amount. The amount used in the ranking is rounded to the nearest $100.
Mathematician here means someone who is currently funded by the Division of Mathematical Sciences of NSF. Also, only those listed as the project manager, what NSF identifies as the PI, are used here.
The amounts listed include funding from DMS and other divisions (DUE is the only other one included at the moment).
The ranking was calculated by writing a short MATLAB code that analyzes the downloaded data from NSF. Although the results were checked closely for accuracy, it is certainly possible that an error was made and this should be kept in minding when looking at the ranking.