Tape block size windows io size

I have noticed that changing the block size on the tape does not change the windows io size as seen by the perfmon counter average disk bytes/read when files are being read for backup on a 2008 R2 server.  I see the same 64k bytes/read when the tape block size is set to any value ie 64k 128k 256k  Is this expected behaviour or should the block size being read match the size being written or does backupexec have a fixed application io size when reading files for backup??