DBV checks Oracle datafiles to ensure that:
- The datafile has a valid header.
- Each datablock in the file has a special "wrapper" which
identifies the block - this "wrapper" is checked for correctness.
- DATA (TABLE) and INDEX blocks are internally consistent.

Since your Standby Database is runing in Mount mode just after applying the redologs there may be possiblity datafile's blocks take times to wrapp up and thus causing dbv to throw an error. Not sure though Pls check with Oracle.