well generally its true la..cos singapore speed limits really quite slow and its acceptable for cars to exceed the speed limit by abit as long as they pose a danger to other motorists. so mostly tp wont catch you if you exceed by less than 20 la, but of course this doesnt apply all of the time. if you suay you still will get caught by tp.
as for speed cameras, i think same rule applies. you see so many cars on right lane of expressways going 100+ km/h, obviously tp no time to catch all. only those that speed really exceed alot (ah beng racers

) will get summon bah.