Impact of period lot size on planning time fence

Hi All

Can anyone explain why does the period lot size setting of a material affects the planning time fence of the same.

For example,

Material period lot size is MB(i.e monthly lot size) and the planning time fence is say 10days and suppose the MRP is run today(14.07.2008) i see that the end of planning time fence falls as 01.08.2008 instead of (14.07.2008 plus 10 working days i.e 28.07.2008).