It does as the MSDN says,

ms-help://MS.SQLCC.v9/MS.SQLSVR.v9.en/uas9/html/5193c976-9dcd-459c-abba-8c3c44e7a7f2.htm

ISNUMERIC returns 1 for some characters that are not numbers, such as plus (+), minus (-), and valid currency symbols such as the dollar...