Error converting string to number

I am working on an Flash application that reads data from XML files. I ran into an issue where when Actionscript converts strings to numbers, it returns values different from the initial numeric string. For example:

Original string: 90330135147081210
Number(“90330135147081210”) returns 90330135147081220

I cannot figure out why it does it. Does anyone have any idea. I tried compiling under both Flash CS3 and CS4, and it always converts incorrectly. Please help.