Different stronger action on the 338L.

Savage would not take the liability of throwing something together.
You need to talk to SSS here as he is the expert and can explain exactly what they did.

Third hand but I read that SSS has seen the effects of running a 338 Lapua on a normal magnum action and you end up with lug setback.

Bolt thrust on a Lapua Magnum is around 25% increase over a typical magnum.