Matilda (de Hastings) Pecche - Missing generation?

Katherine Atherton Writes:

Looking at the family line from me to them, they both come up as 21st grandparents and 24th generation. However, they fall in line with the 22nd grandparents and the 25th generation on the chart. It is off by one generation. I cannot find the error. I do not want to change anything without knowing the exact problem. I would appreciate the help. Thank you,