forecast combinations

We ran shipment history collections and EP_Load twice. The first time we collected shipment history for 100 item-location combinations. The second time, we collected 50 combinations. After the first time collection, we did not run forecast engine. After second collection, we ran the engine. After two-time collections, there are 150 combinations loaded. I was tod the engine started after the second collection would only forecast 50 item-location combinations collected from the second collection.The 100 combinations from the first time collection would not get new forecast. Is it true?