Most manufacturers provide specs without sufficient context to be very useful. Naim probably rated their amp conservatively
and is perfectly fine driving 4 ohm speakers...plus, the speakers may be rated as 4 ohm nominal and still be an easy to drive design. JA's measurements always flush out the important characteristics of a component when considering compatibility issues.

You can sometimes get away with some serious compatibility issues if you don't listen at very loud levels, so the consumer has a roll in the equation as well.