Does Explorer calculates folder sizes wrong ?

I recently added a Raidsonic NAS900 Harddrive to my router with the goal to
backup various files from my PC. To copy/sync files from my PC to the NAS I
use MS SyncToy.

Problem is when I compare the synced folders in Explorer I get different
sizes and amount of files, while it should be extactly the same. For example
Explorer reports 8799 files on my local HD, but only 7670 on the NAS.

First I thought SyncToy simply wasn't copying everything over, but I tried
several times (even with other sync software, such as Allway Sync, DirSync)
with no errors reported. Far more strange is the fact that Explorer often
reports even different folder sizes/amount of files on the NAS when
recalculating.

Does anybody know what's going wrong here ?

Thanks for your help in advance,
Günter