Different size after zfs send receive

[kc atgb]

Hi,

Some days ago I had a need to backup my current pool and restore it after pool destroy and create. 

The pool in my home server is a raidz1 with 4 disks. To backup this pool I grabbed two 4TB disks (single disk pools) to have a double backup (I have just one
sata port left I can use to plug a disk). 

The whole process of backup and restore went well as I can say. But looking at the size reported by zfs list make me a little bit curious. 

storage/datas/ISO                                                        35420869824  381747995136    35420726976  /datas/ISO
storage/datas/ISO at backup_send                                                 142848             -    35420726976  -
storage/datas/ISO at backup_sync                                                      0             -    35420726976  -

b1/datas/ISO                                                        35439308800  2176300351488    35439210496  /datas/ISO
b1/datas/ISO at backup_send                                                  98304              -    35439210496  -
b1/datas/ISO at backup_sync                                                      0              -    35439210496  -

b2/datas/ISO                                                        35439308800  2176298991616    35439210496  /datas/ISO
b2/datas/ISO at backup_send                                                  98304              -    35439210496  -
b2/datas/ISO at backup_sync                                                      0              -    35439210496  -

storage/datas/ISO                                                        35421024576  381303470016    35420715072  /datas/ISO
storage/datas/ISO at backup_send                                                 142848             -    35420715072  -
storage/datas/ISO at backup_sync                                                  11904             -    35420715072  -


storage/usrobj                                                            5819085888  381747995136     5816276544  legacy
storage/usrobj at create                                                         166656             -         214272  -
storage/usrobj at backup_send                                                   2642688             -     5816228928  -
storage/usrobj at backup_sync                                                         0             -     5816276544  -

b1/usrobj                                                            5675081728  2176300351488     5673222144  legacy
b1/usrobj at create                                                         114688              -         147456  -
b1/usrobj at backup_send                                                   1744896              -     5673222144  -
b1/usrobj at backup_sync                                                         0              -     5673222144  -

b2/usrobj                                                            5675188224  2176298991616     5673328640  legacy
b2/usrobj at create                                                         114688              -         147456  -
b2/usrobj at backup_send                                                   1744896              -     5673328640  -
b2/usrobj at backup_sync                                                         0              -     5673328640  -

storage/usrobj                                                            5820359616  381303470016     5815098048  legacy
storage/usrobj at create                                                         166656             -         214272  -
storage/usrobj at backup_send                                                   2535552             -     5815098048  -
storage/usrobj at backup_sync                                                     11904             -     5815098048  -

As you can see the numbers are different for each pool (the initial raidz1, backup1 disk, backup2 disk and new raidz1). I mean in the USED column. I have
nearly all my datasets in the same situation (those with fixed data that have not changed between the beginning of the process and now). backup1 and backup2
are identical disks with exactly the same configurations and have different numbers. I used the same commands for all my transfers except the name of the
destination pool. 

So, I wonder what can cause these differences ? Is it something I have to worry about ? Can I consider this as a normal behavior ? 

Thanks for your enlightments,
K.