The engines aren't quiet the same.
The E36 M3 had two displacements and two specification.
The E36 M3 first came out with a displacement of 3.0L, where the European models had 286 horsepower and 240 for the U.S. models.
Later (forgot the year) BMW changed the displacement from 3.0L to 3.2L, and the European models also had variable valve timing, and bumped the horsepower up to 321. The lack of variable valve timing for the U.S. models kept the horsepower rating at 240.
The current 3.0L engine used in 3 series is rated at 225 horsepower (SAE) for U.S. models and 231 (DIN) for the Europeans.