Why is "exhorbitant" a common mis-spelling of "exorbitant"?

The answer to why it's such a commonly misspelled word is likely due to the fact that it's been being misspelled ever since its inception as a word.

etymonline shows it originating in the mid-15th century, but there are books that have the misspelled version dating all the way from the 16th century, such as in this French book from 1581:

Legende de Domp Claude de Guyse, abbé de Cluny: contenant ses faits et ...

or from this Scottish book from 1585-1592, which has three instances of the misspelling, so you know it's not just a typo:

Scotland. Privy Council - 1585

Google Books shows the misspelling thriving from the 18th century-on:

An interesting thing that can be picked up from the sources is that in the mid-18th century the misspelling made its appearance in many French texts, and an Ngram analysis of French documents confirms this:

Google Ngrams French

Now is this a case of the British misspelling marking its influence on French soil, or was the misspelling French in origin all along? I doubt the latter, but maybe someone else can shed some light. French was the lingua franca during the 17th century so its possible.

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There are various English words that derive from Latin words beginning with "exh-":

exhonorate is another common misspelling of exonerate, but that has a Latin root beginning with "exh", unlike "exorbitant" and "exonerate".

https://en.wiktionary.org/wiki/exhonorate

My guess after all this evidence is that the medieval Brits thought that "exorbitant's" Latin roots had an 'h' in them, which made it a common misspelling at the time, and that misspelling survived through the ages. Truly a misspelling for the ages.

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