SparkCore uses TI's CC3000 unit (http://www.ti.com/product/cc3000) which is a 2.4GHz wi-fi module.

Flutter is using CC1001/CC1200, which are 900MHz radios. 900MHz (as a longer wave) will travel much farther to begin with, plus the module is designed for longer-range use :)

(Also if you put a 900MHz antenna on a 2.4GHz radio you're gonna have a bad time)