Landlord's Rent Net Income Payment Shortage

Why is there a shortage in the Net Rent Payment received by the Landlord?

We sometimes have to send less Rent net income to the Landlord. This is because some extras expenses were accrued during the month. This includes but is not limited to Property Insurance, Property Taxes, Special Assessments for your Homeowners/Condo Association, Increase in you Monthly Homeowners/Condo Association Maintenance, Repairs, and/or Improvements.