the contractor shall submit one invoice to the client annualy in arrears. each invoice shall be submited in april and shall be submitted for the previous years work
the client shall pay the price of the services each month in arrears and based on 1/12 of the current years contract price.
from what the others have said I think you are reading this wrong, a simple call will clarify though,
But I take that as already suggested you supply an invoice in april, but you are paid monthly in the mean time at the end of each month.
It make sense as it cuts on administration as it is only one invoice to deal with per year per contractor