That is one of the strange yet bedrock principals of the hobby: in many cases there is an exponential price increase for an infinitesimal (or virtually invisible) increase in card quality. For example, I could probably not see a difference between Evan's '52 Mantle 9 and a 10 (and in one case I think one 10 looks a little worse than Evan's 9), yet the 10 would likely cost a few million dollars more. Then of course there are the cases where the higher graded card actually looks worse than a lower graded card.
Yet in many other cases— when the flip and the card are in harmony— I see no problem with a buyer paying a big price difference if they also see a big difference in card quality. For example, the .5 grade difference between these two cards resulted in a 2x+ premium. So not every case of a higher graded card going for a large premium over a card one half grade lower is the same.
