I really doubt that it was used as a calendar.
I think someone was trying to decipher the text and writing the month names by the symbols makes it easier to look for those names in the surrounding text. That's my gut feeling. I could be wrong.
Also, for those not familiar with Latin scribal abbreviations, writing a word differently each time is completely normal for medieval text. They did that constantly. If a word appeared three times in a sentence, it was sometimes abbreviated three different ways. The conventions for abbreviations were known to all scribes, so it didn't really matter which one you chose for a word, out of the many options, as long as there was enough context to understand it.
I think someone was trying to decipher the text and writing the month names by the symbols makes it easier to look for those names in the surrounding text. That's my gut feeling. I could be wrong.
Also, for those not familiar with Latin scribal abbreviations, writing a word differently each time is completely normal for medieval text. They did that constantly. If a word appeared three times in a sentence, it was sometimes abbreviated three different ways. The conventions for abbreviations were known to all scribes, so it didn't really matter which one you chose for a word, out of the many options, as long as there was enough context to understand it.