The root cause for this difference is that these reports are different and will display different things.
Self billing invoices show commissions that have been included in that specific invoice only.
The payment status report shows commissions based on the date range applied.
Because there can be overlap between the date ranges of self billing invoices it can often be the case that 2 commissions from April can be paid in different invoices.
This may cause some confusion when looking at an invoice and comparing it to commissions from a date range.
You can easily check this by looking at the payment status report and checking the self billing invoice number. If they are different then the commissions from that period have been allocated to different payment months.