[1][2] Net sales essentially refers to the same thing, in absolute numbers.
Before the rise of the direct market, American comic books were sold to news agents through distributors in anticipation that a significant portion of the print run would eventually be unsold, then returned to the distributors who invalidated each returned copy by deliberately damaging the cover for return to the publisher for refunds where the material was pulped.
Thus the business goal of the publisher would be to attempt to minimise those returns to maximise their income.
When Marvel Comics came to prominence under Stan Lee's new artistic direction in the early 1960s, his publishing line by title quantity was highly restricted because their distributor was Independent News, owned by National Periodical Publications (Now DC Comics).
That meant that while Marvel's product might not have sold as much as National's in simple numbers, Marvel had to refund the distributors significantly less money for unsold product and thus was becoming much more profitable.