The SAE Certification standard was revised halfway through last year.
The changes to the motor test program were made to more accurately reflect real-world conditions.

In order to advertise an "SAE-certified" horsepower rating, Acura chose to re-certify their motors using the new test procedure. If you look around, you'll notice that many other manufacturers chose to re-test as well.. with a similar reduction in the HP rating.

With the 2006 model year RSX, the motor internals were not changed.
The lower HP number is entirely due to the changes in the SAE test procedure.