Most forgeries were not of rarities and high value stamps like Sperati's operation.
Printing was cheap back when, particularly quick and dirty bad printing. The French Colonies Navigation & Commerce/Type Groupe forgeries here, now attributed to Hirschberger, are good quality but were probably still offered relatively cheaply vs. genuine. Plus a collector would also think they were getting a complete set with no effort.
There are Japan forgeries of the imperf 1946-7 issue, where the genuine are still minimal value stamps today. Low values of that issue were forged for the packet trade. So the forgers and packet makers both made some money.
We have the modern US coil forgers on eBay
and elsewhere today. One 55c stamp is cheap, but with 100 in a roll and an offering of several rolls at once, things start adding up. And the suckers really like saving all that money vs. face value without caring or suspecting anything might be wrong.
I don't know how you cataloged the first two New Caledonia stamps.
Look again and you'll find the fake overprints to be on the wrong stamps.