I think the confusion is with ahetuka birth and ahetuka cuti-patisandhi.
Monkey bhava was an ahetuka birth DUE TO an ahetuka cuti-patisandhi from a PREVIOUS bhava.
Then a tihetuka cuti-patisandhi occurred when the kammic energy for the monkey bhava was exhausted, and then a tihetuka birth in the human realm took place.
I also revised my previous reply.