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

The spring is just there to hold the check ball assembly in place. It sounds like the OP has some lifters with 'check plate' instead of a check ball and the spring is different for that particular internal design. Neither spring should have anything to do with actual hydraulic lifter operation; the oil trapped by the check valve of either design does the work as noted.

I actually don't think the different pressures on one side or the other have anything to do with it... (but there are some strange things done out there). The idea that some lifters were replaced at some time with a different brand makes the most sense.