relations between due date&ending date in MPS/MRP

Now one end user complain the interval between due date and ending date of Sales Order and Forecast is different after MPS calculation. In our system, the interval are normally 1 week. But this time we found the interval for some forecast change to 3 weeks and the interval for SO keeps 1 week. I read some document and realzie that safety lead time and default safty lead time in Mfg setup will affect the interval, but we didn’t setup any safety lead time. Howerver I am not sure how to define the interval 1 week in which repport or codeunit and other else setup can cause the interval? (navision attain 3.01B) Any ideas? Thanks in advance.