The review I am guessing is using the default clockspeed of both processors.
If that is true, then the 8700k has a Turbo Boost of 4.7Ghz while the 7700k has a Turbo Boost of 4.5Ghz.
Good reviews will attempt to run similar clocks on both processors. I strongly believe that other than the 2 extra cores on the 8700k, the IPC of Skylake/Kaby Lake is largely similar other than cache sizes and some IPC improvements. Those improvements alone is not a significant contribution to FPS in games. More so for overclocking and video encoding...
So assuming that both 7700k and 8700k are clocked at 4.5Ghz or 4.7Ghz equally, I would not be surprised to see absolutely no gain at all in games. They should be within +/- 3-5 FPS at most which is nothing. Just look at Skylake-X, they perform worse than the 7700k in most games largely due to the superiority of clockspeed on the 7700k. More cores does not necessarily translate into better gaming benches unless games are specifically written to take advantage of those cores. It will take a little more time before we really see a good use of cores in games thanks to Intel being dick about 4 cores being enough until Ryzen decided to up the game.