Help> Interpretation of MB5B result

hi Expert,

using tcode MB5B for material XXX in plant P1 and a storage location SL1 between two accounting dates Date1 and Date2 we found that:

1- stock at the Date1 date is 10.

2- stock at the Date2 date is -1 .

knowing that we didn't authorized negative stocks , how SAP allows to have this stock -1 on this Date2 date??

cordialy

Said