Shared WAL archive between master and standby: WALs not always identical

Hallo,

I am trying to setup shared WAL archive between master and standby. Standby
is synchronously streaming from master and both servers run with
archive_mode = always. The ideas is that when promoting standby to master
we would not missed WALs.

My problem is that sometimes WAL uploaded from master and from slave are
not 100% identical. In most cases they are but occasionally they are not. I
have written small script that ensures that upload is free of race
condition and I log md5 sum of each WAL. Aren't WALs from master and
standby supposed to be identical? After all, standby is just consuming WAL
that it is receiving from master ...

Or do you have any better suggestion on how to achieve continuous
incremental backup?

Thanks in advance