Bear in mind that everywhere else in the world gets 1 mph increments for ACC. So Lexus chose to do this for Europe only. Also, every other car in the world uses 1 mph/kph increments.
If I’m driving in a 50 avg speed zone for 20 miles and I want to gently overtake someone (as happens all the time), I don’t want to jump to 55mph in one flick. In fact, that’s insane, stupid, and dangerous.