ISm band interference?

Hi everyone,

I do a research on paper "Design of Multple Stopband filters for interference suppression in UWB applications and found this sentence inside..

Except for ISM band interference,which varies in frequency from 5 to 6 GHz with a bandwidth of 100MHz,all other frequencies are located at fixed frequencies.

Do anyone know or have any scientific verification that ISM band interference varies in frequency from 5 to 6 GHz? As i know,ISM band that i know is 2.45,5.2,5.8GHz only.

Thanks,
Chai Ling