Why do they have to be copying errors?
In many languages, there are combinations that are common and variations of the same combinations that are rare. It doesn't have to be one or the other. It might be a common combination that also has some rare variants.
Take for example, the loanwords I mentioned in the previous post. In English "qu" is a common combination, but there are loanwords with "q" without "u". Loanwords were not at all uncommon in medieval texts and spelling variants were very common. Plus, the way things were abbreviated, common combinations might turn into rare combinations if they were abbreviated only once in a while (this is not uncommon either, the word might be spelled out most of the time and then suddenly the scribe abbreviates it, either due to haste or lack of space).
Even if it were a synthetic language, common combinations with rare variants might occur.
In many languages, there are combinations that are common and variations of the same combinations that are rare. It doesn't have to be one or the other. It might be a common combination that also has some rare variants.
Take for example, the loanwords I mentioned in the previous post. In English "qu" is a common combination, but there are loanwords with "q" without "u". Loanwords were not at all uncommon in medieval texts and spelling variants were very common. Plus, the way things were abbreviated, common combinations might turn into rare combinations if they were abbreviated only once in a while (this is not uncommon either, the word might be spelled out most of the time and then suddenly the scribe abbreviates it, either due to haste or lack of space).
Even if it were a synthetic language, common combinations with rare variants might occur.