I found what appears to be a complete list (several lists actually) of GM RPO codes and it shows that the RPO L78 was a code for a 400 cubic inch V8 with a 4 barrel. Looking up just L78 RPO it clearly was a big block 396 with forged crank, solid lifter cam, and cast aluminum hi rise intake.
Does anyone know the reason for the discrepancy?

They did make a 400 small block that was externally balanced at the flywheel and harmonic balancer, working on them back then you had get the correct parts. No sure on the RPO code though.

A 400 big block Chevy is a 396 bored. 030" over.
They had a bunch of blocks that got left outside and rusted the cylinder walls.
Instead of scrapping them they were bored .030" over.

GM is constantly reusing, redefining and repurposing RPO Codes. One engine goes out of production and it's RPO code is archived for a few model years. Later, the RPO code is brought out of archive and used for a new engine.

LS7 was originally destined for a big block 454 that was cancelled for production in 1970. GM did release parts as over the parts counter items and in 1974 did offer a complete LS7 advertised for off road use and marine use only.

And the LS7 is now in the LS series engines again.

While the L78 engine is often associated with a "396" designation, in 1970, the final year of production for the L78, the engine was actually a 402 cubic inch engine due to the 396 block being bored out slightly, making it effectively a 402 despite still being marketed as a "396" on the car.

Key points about the L78 and 402:

Displacement change: The L78 engine in 1970 was essentially a bored-out version of the 396, resulting in a 402 cubic inch displacement.

Marketing discrepancy: Although the engine was technically a 402, Chevrolet continued to market it as a "396" on the cars.

Last year of L78: 1970 was the final production year for the L78 engine.

The 400 was really a 402, Chevy just marketed it as a 400...