Googling actually lead me to the NMO Admin Guide. Here is an exerpt:

"Save set bundle join
Two save set bundles are created by separate level 0 backups of files A and B. 
Then a
new backup set is created by a level 1 backup of both files A and B. Since the 
new
backup set is dependent on both of the preceding level 0 backups, NMO combines 
all
three backups into the same save set bundle.

If the original file A backup has the oldest backup time, NMO places the new 
backup
set (from the level 1 backup) into the save set bundle of the (level 0) file A 
backup.
NMO then moves the original (level 0) file B backup into the save set bundle 
with the
other two backups. This method of combining existing bundles into a new save set
bundle is known as a ?save set bundle join.?"

This obvously points to NW's Save Set Consolidation feature - However, i don't 
know whether it works similar with a 'normal' filesystem backup.

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