f and p appear predominantly in the first lines of paragraphs

[Jorge_Stolfi]

I can imagine there might be some difference in word choice in the top lines and that this would cause some degree of distortion in the stats.  But I struggle to believe it could be a decisive explanation, given the size of  the /p/ dominance.

The digraph "th" is very common in English because it occurs in the very common words "the", "this", "that", "there", "their", "them", "thus", "then", "than", etc.  But these words rarely occur at the beginning of a herbal entry.  Therefore I bet that the frequency of "th" in the head lines of English herbals is dramatically lower than its frequency in other lines.

Being shorter than average, those words will also be less frequent at the start of a line than elsewhere in the line; and more common among the last 2-3 words of a line.  Thus I expect that the frequency of "th" will have a similar profile along the line.  No matter how wide or narrow the pages are.

(These bets assume that the text is formatted with the trivial line-breaking algorithm, without hyphenation, as in the VMS.  They may not hold for high-quality typeset text where line breaking and hyphenation take grammar and semantics into consideration.)

All the best, --stolfi