Why Backlog management planning gives a planned date even though supply exists

Summary:

We're testing BM in one of the lower environment and observed an unexpected behavior.

Content (please ensure you mask any confidential information):

SO with item AB1, ship from warehouse = 103. No hand in AMS but onhand is available in source org 310. (Sourcing rule is defined as transfer from 310 and assigned to the item category)From 'Check Availability', it gives a resonable date with considering the onhand in source org but in Backlog Management, it gives a new planned date in far future which doesn't make sense.

Why backlog management doesn't repect the ATP rule and sourcing rule as GOP does?