It would have helped if you wrote which camera model you have, but here's my best guess:
Your camera does not have an iris diaphragm. Older models like from the powershot A4** line or the IXUS / Powershot SD* series do not have iris diaphragms, they have a ND filter instead. That means the size of the aperture is fixed, the camera controls the amout of light by either swinging in or out the ND filter. Hence the fixed 2.8 or 7.1.
