What factors affect your flash exposure? There are four:

1. Aperture
2. ISO
3. Flash power
4. Distance from flash to subject

Two that do not affect flash exposure, or affect it to a lesser degree:

1. Shutter speed
2. Distance from subject to camera

What factors affect your “ambient” (available light) exposure?

1. Aperture
2. Shutter Speed
3. ISO

Several that do not affect your ambient exposure, or affect it to a lesser degree:

1. Flash power
2. Distance from flash to subject

So you can see that by altering shutter speed, you only affect ambient exposure, while by altering flash power you only affect the flash exposure.