I guess cats are like people when it comes to moods and personalities. Some days the boys are just running all over the place; other days (like Sunday) they're more quiet and sleepy, at least when we're around. Most of the time they alternate between periods of wildness and periods of sleep in the same day.
I have to admit Arthur is much cuter this way than when he is hiding under the couch or trying to chew the litter liner or messing with daddy's computer wires.
What's strange is that you can never tell which cat is going to be aggressive. Today, it could be Arthur. Tomorrow it could be Beowulf who is leading the way under the sofa or chewing through a wire.