INVOICE

here I created PO , qty 20

1.GRN 10qty -posted 2.GRN 10qty -posted

MIRO 10qty , in this document when i have checked reference document no., it found the material document no.of GRN no.1

again when i m going for MIRO qty 10(remaining).at this time i found the reference document no.is the same no. of material document of no.1 instead of GRN no.2

how this could be possible?suggest me