The tier list doesn't depend on the skill of two specific people playing in one match, it is the combined effort of many players over time and cross referenced information. Whether or not those players use everything that is available to them doesn't change how the characters fall against eachother in terms of effectiveness. A tier list is something that changes less and less over time, and at this point, it's pretty definite.
It really seems often that I see people try and discuss a tier list, and don't actually understand what it is or why it's there, and then when it is explained to them, more often then not the explanation is ignored and they continue to think how they want, and never get it. It effectively combines statistics and probablity into something you can almost put money on, with a margin for error. If someone discovers something new that can help a character defeat others more easily, his tier level might go up, unless of course it only helps him defeat characters who were already below him.
In the case of Reptile, he has had the biggest move on the tier list in the last several years. People are starting to use him and his strats have become more acknowledged. That's another big part of a tier list. If most players who pick up Reptile can't master his juggles or his zoning, that means he's too hard to use, and ultimately not an effective character. Since this is kind of a 50/50 situation for him, he is still ranked high, however has the potential to go higher still.
There are enough people who use Kabal and Human Smoke to cement their spot in the top 2, and comparatively, Reptile's effort to win is so much greater than the others even though they are all in the top 5. A character like Cyrax can perform 90-100% mid screen combos if you can set them up and perform them, which might make people say ""Wow that should make him top tier" but how often do you see these combos happen? In the hands of an amazing Cyrax player, you might see a 71% combo from time to time, but that's pushing it, he's just too difficult for a person to use. Getting 52% with Reptile consistently is a lot easier than getting 52% with Cyrax, not only in performing the combos, but getting to the combos.
For a tier list you can't just say "infinites will win" (ie: Kung Lao universal infinites > everyone) because characters being used at their best doesn't mean performing an infinite once you get it, it includes getting to that infinite which is far more complex math and reasoning than I would care to have a computer look at. A computer might look at that way, but there's a chance the infinite will never be possible to perform, so it is as if it doesn't exist to begin with. It's harder for Kung to get the infinite on Kabal or Human Smoke being played at their highest capacity before Kung is defeated, or, there are less opportunities against them.