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.
There is a huge thread on yellowbullet about caltracs and similar devices. Several guys (I think they were Stock racers) said they went from conventional leaf springs to a single mono leaf and a split mono and they didn’t see anything.
Once you bolt that bar on the bottom of the spring you effectively now make the spring do nothing but hold the car up.