PO dates difference

Hello Experts,

There is a PO Availability  date difference in APO and R/3 system the procurement type is external

For the only one PO number there is a mismatch in Avail Date for remaining PO are fine  for particular  material

Example : APO 28.03.2016 in R/3 02.04.2016

Can any one suggest me what will be the reason.

Thank you