I don't understand either. But what I heard from a Calvert rep at the races (winternats and world finals) is that the combination is not recommended.
Edit: my guess? The very stiff front segment restricts movement and leverage, of the caltrac bars.
I’ve been running them with SS springs for a couple of years, no problems.
Have a set of their mono leafs in the spare bedroom that I picked up, thanks to a friendly member with Summit discount coupons. Will swap them out once I see how the new motor and Dice converter works out so I can compare any improvements.