This is my opinion on the matter. By no means am I claiming this theory is fact or that you have to abide by it.
The issue: Corythosaurus was claimed to have been cloned illegally in 1999 by the Dinosaur Protection Group, but the InGen hunters in The Lost World: Jurassic Park had info sheets on the species, meaning they were intending to look for this animal on the island. How would InGen know about the animal if they hadn't cloned it yet?
My theory is that Corythosaurus had been cloned prior. On InGen's list, Corythosaurus DNA is shown to be at 97% completion. It's not 100%, but this doesn't need to be the case. If you look at the clean-up list in this report on the DPG website, you'll find Herrerasaurus was active on Isla Nublar with only 60% DNA completion. This indicates InGen could clone animals with as low as 60% DNA completion, but of course they would strive to get 100% in order to have less "flawed" animals (which harkens back to a novel element with the "versions" of the animals). This would allow InGen to have early clones of the Corythosaurus available for observation, giving them data to put on the information sheets the hunters received.
The asset catalogue ("InGen's list") on the DPG website was updated December 1996 with October 1994 Nublar data and 1993 (unknown month) Sorna data (that's why the 1994 cleanup is referenced throughout). This would indicate that by sometime in 1993, Corythosaurus was not considered an active species on Sorna, which is corroborated by the cleanup page claiming it was currently and previously an inactive species. So why include it in the Site B specimen packet? I think the simplest answer is that InGen was not being very thorough, as suggested on the Jurassic-Pedia page here. Ian Malcolm's group didn't have a digital informational slide on the Tyrannosaurus rex, despite it being seen on Hammond's computer, which had recent readings of Sorna's ecosystem. You'd think even if the plan was to stick to the outer rim of the island (for both teams), info on the Tyrannosaurus would be handy regardless, given it was the top predator on the island at the time. The species packet given to the hunters having Corythosaurus in it when the species wasn't active does seem unnecessary, but I wouldn't put it past InGen to either be a bit sloppy (read: rushed) or to overcompensate.
I considered that InGen simply cloned more Corythosaurus between 1993 and 1997 and the catalogue didn't have updated data, but as of current lore, it would be impossible since they completely abandoned the island until late 1998/early 1999.
In the end, it really doesn't bother me like it bothers some other people. Out-of-universe, it was an oversight as any human being is susceptible to doing. In-universe, it's passable as something InGen could conceivably do.