It would be interesting to find when the first written material used the word 'bunkai'. An early reference is Mabuni's "Sepai no Kenkyu" pub. 1934.
The choice of words for the title: "kenkyu" which means 'study' or 'research'. In the book are pictures documenting the solo sequence and then corresponding application interpretations. There is no mention of analysis methods, only documentation of specific application interpretations. Interesting he didn't chose 'bunkai' or 'oyo' for the title.
Yet, the translated text of Seipai no Kenkyu in 2003 by Mario McKenna captions and references the applications as 'bunkai'. If he re-translated the text today, would it become 'oyo' ?
-or- did Mabuni himself use the term 'bunkai' to caption his photo's? Most readers don't have access to the original, nor can we deciper the kanji easily so the trust lays within the translation.
I have several translated works published and unpublished (one translated from French
) which use the term bunkai in reference to specific application. The word application and bunkai are also used interchangably.
If Mabuni used 'bunkai' to depict specific applications, then there is basis for stretching the term as such.
however, if it was mistranslated, then I suspect that 'first' mistranslation goes back well before 2003 and past the early 70's. Why do I think that? I have the original test/syllabus of our Goju school which was printed in the early 70's. It refers to bunkai and the way they meant it was for testing - by demonstrating specific applications. (although the term bunkai itself was not used in class. they never said 'pair up for bunkai' they always said 'pair up for kata drills' or similar).
so to answer the question, why is 'bunkai' translated into English to mean 'application' ? It appears to me that translators and people bringing Karate westward prior to the early 70's got it wrong - and it was never corrected or considered up until recently.
and 'oyo' I've never heard used or printed until 3 years ago.
'bunkai' means decomposition (implied: for analysis).
'oyo' means application. or the Aikido use: 'oyo-waza' meaning applied variations of a base technique.
personally, I like using clear first language, so "kata drills" and "kata analysis" works best for me. It was mentioned to me that Matayoshi Shinpo didn't distinquish the two when he instructed the students to "play". trying variations based on principles. or similar in meaning to Aikido's 'oyo-waza'.
Perhaps prior to the invented language of karate terms and technical categories with structure, maybe the process of drill+analysis was within the same - the students just played with possibilities based on responsive interaction. as oppossed to separating out the two, leaving the student with static and dictated "if they do this, then I'll do that" application.
To use your guitar metaphore, a student learns chords and basics - then they play with those. analysis and drill are within the same act of playing. and 'analysis' doesn't have to mean cerebral. There are other senses which can analyize. best summed up in the phrase: "getting the ear and feel for guitar." vs. "mimicing a song, note for note".
I'm not a historian - just "playing by ear" hoping to add to the discussion.