The Nimbus 2000 was fastest broomstick during Harry's first year at Hogwarts and the Nimbus 2001 was fastest in the subsequent year. It's safe to assume that the number in the name represents a year.
Harry used the Nimbus 2000 in 1993 and Malfoy used Nimbus 2001 in 1994. Why weren't they named Nimbus 1993/1994 or Nimbus 93/94?