What's best and easiest to check sync result ?

[SYNass IT Ubuntu / Linux]

Hello Ubuntuans, 
Recently we discussed rsync quite intensively ! ;-)
Since I was experimenting with it was very interesting to hear and
learn !! ;-))

I already have done some "sync's" from my HDD to an external HDD quite
sucessfully and 
I only have some questions how to check and control the result best and
easiest.

My rsync closes with this statement:
sent 7565493096 bytes  received 1457080 bytes  819865.67 bytes/sec
total size is 7557361145  speedup is 1.00

and properties shows following for:
Source: 4054 files / 6GB
Target: 4054 files / 6GB

So, Source and Target, show identical values ! ;-)
Why is rsync's closing statement showing a difference ? :-|

How can / does one check exactly these difference ???
Well, there are diff or Meld but ways to extensive and 
very difficult to discover the difference and its reasons, 
i.e. their files ??

Is there a better solution available ?

TIA for your advice and guidance, 
cheers, svobi



PS - My command:
rsync -av --progress /source /target

An extra question for parameter "Backup":
How and when does one need this being set !
Only at the first time to get a full set ??

The following link shows a dsicussion about sync's options:
http://linuxgazette.net/147/misc/lg/rsync_options.html

Unfortunately "Backup" isn't mentioned. ;-(

Once more TIA for your help. ;-)