Maybe, the voynich manuscript is not a book but a compendium of different works merged as a one. In that case you are seeing already a lot of diferent manuscripts that have the voynchess; they just happen to be clustered in a single item, cause someone a long time ago decided to collect all instances of the dialect: especially if it is like a phonetic transcription because such dialect has no writing words.
In today world we have tech to record the natives of a dialect, in medieval times not so much.
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In wich case it may be the only attempt to register or transcribe the source dialect from its oral form to an existing writing system, and what if the writing system is from a dead dialect also; we may be seeing a double combo or the mystery of a mystery.
That could explain why the voynichess letters can be identify but not the sintaxis arrangement.