I saw this mentioned in the group chat earlier, and booted up the PC and PSP versions to compare.

In the PSP version, the DLC stages that have fights all have fights against level 100 units. The stage bonuses reflect this. In the PC version, those bosses now scale with your level... but the bonuses are unchanged, allowing you to get really powerful stuff really early on.

Example: If you were to start a new game and go right into the DLC stages, you'd be fighting a level 2 boss but getting level 100 loot.