Total disruption months = sum of the number of months that each of the disruptions can occur in any given year. For example if you have three disruptions and the first can occur in 3 months during the year, the second in 5 months during the year and the third in all 12 months, the total disruption months equals 20 = 3 + 5 + 12
You can change the number of disruption months by going to the edit menu and selecting Advanced User Parameters. The default for disruption months is 25. You can set it as high as 50.