As you can see from the schematic, it's the combination of pot and capacitor selector that produce the wide range of oscillator control:
But the challenge with the classic 555 astable circuit is trying to hold frequency or duty cycle constant while adjusting the other. This circuit doesn't solve that problem, but for something different, I plotted the functions with WolframAlpha. If you correlate the two graphs below, you can see the severe penalty you pay in terms of duty cycle when attempting to push for maximum frequency. It definitely is a case of trying to find the best compromise for your application.
As always, all notes, schematics and code are in the Little Arduino Projects repo on GitHub, including live links to WolframAlpha to reproduce these plots.
Frequency for all values of R1, R2 (C=1µF)
Duty Cycle for all values of R1, R2 (C=1µF)
