Why are hydraulic roller lifters, different on different sides in the engines?

Alright! I thought the noise was from sludge & that's why the engine went apart to star with but now I understand the plate or ball or its seat might be worn, right?
The plate in those that had plates do have groves from the springs, so I reckon the "ball-ones" are the ones to prefer, no trace of wear on them at all...
So now I & you know, all of us who's been scratching our heads... :)