There's a lot of subtlety in the rendering pipeline. Do texture samples that come from cache lines not count because they're not being read from GDDR? Do values interpolated from polygon vertexes not count because they aren't computed from scratch at every pixel? We have approximations left and right, but you're absolutely going to draw the line at using perfectly good pixel data from the prior frame as a shortcut?
CBR isn't as accurate as native rendering, but it can be a staggeringly close approximation. It can also fall back to interpolation when the prior frame isn't a good reference, as during rapid motion, at frame edges, or during cuts between scenes or camera angles. Worst case, 50% of the pixels are rendered natively. Best case, 100% were rendered through a conventional lighting pipeline, just not during the last frame.