Heh.
To answer seriously: because we are discussing tablature for (primarily) stringed instruments and Gregorian chant is a vocal form. All of the examples I gave are of tablature for closely-related stringed instruments -- three are for Renaissance lute and one is for Renaissance vihuela, which was generally tuned the same as the lute.
All of the examples are contemporaneous. They illustrate the point that tab is not necessarily intuitive because it was not/is not standardized, even among people writing for the same instrument at around the same time period. If you were a student of Mudarra you could read his tab, but probably not John Dowland's tab; and vice-versa.
As already noted, this situation, although somewhat improved, is still true today. Staff notation is standardized; tablature notation is not. Different publishers use differet symbols to mean the same thing, and sometimes the same symbol to mean different things. Without actually hearing the piece first, you can't really be sure of what those ambiguous tablature elements mean.
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