Most role-playing rule-sets confuse narrative and strategic differences. In an attempt to make the experience of playing a different "type" of character feel different they introduce a multitude of rules to govern the same actions.
Players and game designers imagine differences that do not exist. Most role-playing games use character build options that "break" existing rules, or add special circumstances to specific actions for characters of a selected background. Unfortunately, this is nothing like how a real world works. Suppose that my character spent twenty years training to the top-level in a specific martial art, should I expect that my character gets some special abilities or rules? If I do I am an idiot. Training in the martial arts will make me better a few things that all other people can already do: punch, kick, control distance, strike with elbows and knees, evade attacks, throw and push, hold people down, and apply chokes and locks. People who come from a different martial arts tradition don't get a different list. They get better at different items on the same list.
In D&D you give a thief a "backstab" where they do oodles of extra damage in the right circumstance. You bolt on a rule that makes your character "feel" different, even if the rule is stupid. Even if it can't make up its mind as to whether it is a targeting bonus or a surprise bonus. You see if it were a surprise bonus ... shouldn't everyone benefit from it against an unaware opponent? If it were a targeting bonus, should it not get mitigated by armour? On the one hand, you could give a "Thief" a backstab bonus that allows for prodigious damage; or, on the other you could make them good at sneaking.
Doesn't it make more sense for a stealthy character to be better at being stealthy at the sacrifice of their martial skills? They may not be as good with a sword, but they are more likely to sneak up on someone and stab them? It doesn't matter if you are a little less proficient with your attack if it is unopposed. Game designers tend to ignore these issues as "too complicated" when in fact they are quite simple. Complications arise when you try to build on top of an already flawed framework. Getting a rule to fit into D&D requires knowing the peculiar interactions with a whole series of artificially constructed mechanics that have no relation to a real experience. If you are a nincompoop who wants to take umbrage with my use of the word "real" in relation to fantasy gaming, please substitute the words "visceral" or "genuine" or "internally consistent" or "immersive" or even "unjarring."
Really there is a universal set of skills. Everyone can juggle. Some of us just happen to have trouble when you introduce the third ball. Most of the popular RPG's out there like to sell you on complications as to why things are different and don't clear through the rubbish and help you see what is the same. They make two characters who are stylistically different mechanically different. The problem is that the interaction of all the different mechanisms makes the game less focused on the narrative and more focused on deciphering the strategic mechanisms -- mechanisms that are flawed and less sensible.