Bandwidth in usrp_spectrum_sense.py

hi, All
i have not been studying GNU Radio for a long time.Some problems
confused me recently.
In the usrp_spectrum_sense.py, i am confused about bandwidth usrp
analyzes every frequency step. Is it usrp_rate=adc_rate/usrp_decim
control the bandwidth that i reffered to? If i want to change frequency
step into a small one, i can modify freq_step=0.75*usrp_rate? Now there
is a contradiction, the bandwidth is so bigger than frequency step, and
what are stored in m.data after every frequency tune? Maybe i
misunderstand this.

any help would be appreciate.
Best regards.