I think a more pertinent question is, why would a class choose to imitate a class feature of another class?
If we disregard the "because roleplaying", it could be because either their class is penalised and/or persecuted for some reason or the other class receives preferential treatment and/or benefits. Which means that the NPCs within that game world have at least the rudimentary knowledge of class to know that a Witch, for example, weighs as much as a Duck.
Because the rules clearly only capture a small fraction of what is going on in the practice of magic. Imagine that studying and memorizing certain mystical symbols actually gave you supernatural power over nature ala AD&D Vancian Magic User. That still leaves off a vast array of actual activities that real practitioners of magic would be up to. I mean the WHOLE CONCEIT is that massive amounts of study and acquisition of deep knowledge of this magic is what allows Magic Users, and not anyone else except the occasional rogue, from understanding and employing it. So clearly there's a wide range of studying, research, etc that happens JUST TO BE A WIZARD AT ALL. None of this is specifically mandated to be accounted for (aside from 1e's level up rules, but even those are highly abstract). So, yes, just by the rules of the game, any wizard is going to be doing all this activity that has no direct translation to any specific capability in-game. So in our imaginary world of magic there would be a lot of people running around studying, copying, writing, and expounding about various theories and applications of magic, etc.
Now imagine there were also 'sorcerers' that accessed magic in a more natural way through internal power, and 'warlocks' who conjured up and bound higher powers, or supplicated them, and were granted these magics. They also are special, they have levels, they most certainly undertake some sort of activities which allow them to understand and enhance their powers. These activities are, just like with magic users, outside the strictly accounted mechanics of the game, but they obviously exist or everyone would be using the same skills.
How do you know you can distinguish between these things? They'd all be 'studying magic' in some sense, and probably the same arcane principles govern ALL of these types of magic to some degree. So they would all have common references, be able to discuss magic with each other, teach each other at least some core skills, etc. Now through in that there are endless variations of these magical traditions that fall below the level of changing mechanics (IE AD&D's "spell books come in many forms" and such) as well there being various mechanically differentiated types within each tradition (warlock pacts, magic schools, subclasses, etc).
It seems to me that distinguishing between them all in anything close to a realistic setting would be, at best, difficult. Even if you had some idea that so-and-so was an "Academy Trained Wizard" and thus memorized from a spell book every day, cast spells of specific levels and types, etc you could well be surprised because, by gosh, he's actually a Transmuter and he gets some extra spells and casts them at different levels, and can make Philosopher's Stone (in 5e anyway).