It's as the community already believed. The TH upgrade chance depends on the difference between your current TH level and the TH level that's on the mob. The lesser the gap, the higher the chance.
Neither does THF. Thief's Knife is borderline useless. It's like requesting to upgrade Inferno Axe, because no other Great Axe had additional fire damage, and if you want that you lose damage from not using an Ukonvasara. The problem is with thieves and the community who view TH7 vs. TH6 as something holy, when it's absolutely not. TH+1 is not essential to a TH6 THF, no matter which way you spin it, it's not even borderline important. Yet even the people who know how useless it is are worried about it, mostly due to historical reasons and frustration rather than proper reasoning. There is absolutely no problem to be solved here.