Block invoice issue?

Good mng Gurus,

I m creating PO for 100 pieces at 100 INR..

Doing GR...but at time of IR, it is getting blocked..

System is showing the below blocking reason:

"Variances occur in at least one item in the invoice.Therefore, the invoice has been flagged with a blocking indicator which prevents it from being paid automatically".

I am not clear abt the reason for blocking..

Pls help..