Unless the calipers have been very thoroughly gone over to balance travel, and it is a delicate process indeed, to have all the pistons move at the same time is exceedingly rare especially with differential bore units.
Sometimes temporarily sticking pistons can occur if they were retracted without cleaning off as much accumulated brake dust / dirt / bugs / whatever as possible. All that stuff packs up against the dust seal and can cause sticking, and in a worst case scenario can require a complete rebuild.
The indicator of this is excessive drag when the brakes are released: normally you will hear a slight rubbing noise and not much more, but if it takes too much force to turn the wheel or one rotor is getting substantially hotter than the other it's time to take corrective action.
The problem started with me painting my calipers. I think a little overspray was on the pistons when I pushed them in. I took it all apart and cleaned it all up. They are now moving better in and out. When I pump the lever with the caliper off a couple of the pistons come out faster than the others and one barely comes out at all unless I put pressure on the others. I had the grinding noise going on when I get the brakes really hot.