At the risk of going off-topic spelling is one of the reasons experts believe we may have an original Shakespeare manuscript in the play Sir Thomas More. Shakespeare spelt silence as something like *silense *or scilense. The particular spelling used in the manuscript has not been found anywhere else in Elizabethen manuscripts. A few other spellings in the manuscript are also idiosyncratic or rare.
The first Spanish dictionary is Covarrubias’ in 1611, yet Spanish spelling in pre-Covarrubias’ texts appears to me to have been much more homogeneous than the English of the same period. ISTM that there were additional cultural factors at play, whether it was one culture valuing homogeneous spelling more than the other, one language being more straightforward in its morpho-phonetic correspondence (less possible doubts about “how the heck do I spell this”), or something else. Those “same person, same block of text, different spelling” inconsistencies Shagnasty points out are even something I see occasionally in modern English but almost never in Spanish (rarely enough to be something I remember).
Would the fact that Spanish is derived from Latin be a factor? If any early modern Germanists are on perhaps they could tell us if German was similarly haphazard in its spelling.
That’s a factor in providing a relatively easy transcription map. But that’s one factor, I don’t know if and which others will be relevant or what would lead to the same person spelling the same word three different ways in a single paragraph. I know seriously lousy spellers and people with multiple degrees of dyslexia, but IME in Spanish they tend to be internally consistant; in English, not so much. There may be other factors there which were already present and different in the 16th century.