I have a Canon G7X II camera, which has a 24-100mm f/1.8-2.8 lens. Why does DPReview's chart show it having an aperture of around f/8 at a focal length of 100mm?
The actual focal length range of the lens on your G7X II is not 24-100mm, it is 8.8-36mm. Coupled with the smaller sensor (13.2 x 8.8 mm) of the G7X II the 8.8-36mm lens gives the same field of view as a 24-100mm lens does on a full frame (36 x 24mm) camera. Similarly, the aperture of f/1.8-2.8 will behave, in terms of Depth of Field, like an aperture of f/5-7.78 when used with the smaller sensor (this equivalence breaks down at macro distances and distances past the hyperfocal point). In terms of exposure, however, it will still be an f/1.8-2.8 aperture.
For more about why many zoom lenses have variable apertures across their focal length range, please see Why do zoom lenses and compact cameras have varied maximum aperture across the zoom range? and Why does my aperture setting change as I zoom on my DSLR kit lens?