Issue with Compensation cycle's user defined column.

There is a user-define "new hire probation" component in Compensation cycle and the data populates via a fast formula. It calculates the user's time period for that particular cycle for example if the user is in the company for 12 months it will show 12 in that column, and if user is hired in between that cycle it will calculate for the rest of the month and will show the data. So for some user even though they were hired long back it is showing 11 months whereas the expected result is 12 months. And this behavior is only seen