RM Unapply amount double

Hi, In the receivables, there is an invoice that has been fully apply but the unapply amount is still the invoive amount (Invoice amount=$1,862.60, apply amount = $1,862.60 and unapply amount=$1,862.60 - should be $0).

It is not the first time that this customer is having this problem.  Is somebody now how can this happen?

I know how to correct it in the table (I applied it last time they had this problem), but they would like to undestand the reason???

Thanks