This could be due to a stock transaction that has been posted to the Item Ledger Entry table (BC) or appropriate transaction table for your ERP since midnight. AGR takes a snapshot of the data at that point, so if e.g. a purchase order was receipted after the snapshot was taken, it will show in the stock the next time AGR updates data (tomorrow).

Is the ERP stock considering the same location(s) as AGR? You can see which locations AGR is considering in the Locations section of the ERP Settings, accessed via the Navigation bar:

For BC/NAV ERPs, if an Item number is changed, it can cause discrepancies in the Item Ledger Entry table and therefore cause incorrect stock quantities to be presented in AGR