The only Gruntz puzzlez I am aware of that do not seem to have the synchronization problem when multiple Rolling Rockz exist in a path are the 'Standard' Levelz. All of those seem to run at 1 second per Tile ... or slower. I know that all of my CLz (...Soduko..., UphillBattle, etc.) have that problem of RRz 'combining' or otherwise varying the gap between on Rock and the next.
The variation comez when a Rock is turning a corner. I believe that the Rock is prone to slowing down (or speeding up?) just a little at that point if other action takez 'priority' ... the switch from eastward-rolling to southward (or northward) rolling simply does not occur smoothly.