OM ATP vs ATP Inquiry results

So we have an item OPERATOR-12345 which when a scheduler performs an ATP availability check in the OM form they see results that say Request Date = Available Date...so all should be good to assign the date. Now at this point they have not scheduled the sales order line yet. This group is new with ATP so they are also checking via an ATP inquiry in ASCP. They enter the same OPERATOR-12345 with same quantity and same requested date. However the results they are seeing is that there is a 1 day difference in this particular instance and there is