I don't think that what you're calling mastery and proficiency are matters of degree like you seem to be expressing. Ability to communicate can be completely separate from understanding, because (particularly if you're of above-average intelligence) most people don't understand things the same way that you do.
I don't think that the underlying inherent knowledge that accompanies the capacity to successfully transmit understanding can be set apart from a more complete understanding.
For example if you can produce any number of outcomes in a single language but lack the capacity to do the same things in another language there's a level of abstraction that you simply have not witnessed which itself gives greater dimension to the fundamental capacities that are being exercised.
If all you know is English, even if you speak it very very very well you can go your whole life missing out on seeing language itself rather than just a one dimensional perspective on a single language. You can have all the parts held in theory but until you can see equivalents in another language in action you lack understanding of English.
The mere capacity to communicate something real demands an empathy that speaks to experience on a higher level. If you can't, in some degree of abundance, find analogous material from which you can more clearly communicate your realm of knowledge to the uninitiated then it says something about the true depth of your knowledge.
Just as you can't get real depth perception without a plurality of vistas neither can you truly see what you're doing unless you have experience, and thereby the capacity for empathy and making links with that empathy, from far outside of your field of focus.
