Why does a Solenoid driven with intermittent duty cylce produce higher force than with continuous drive?
You are missing the fact that the bottom lines are for the two different types.



Figure 1. The yellows are related. The blues are related. [Colouring mine.]

The problem with solenoids is thermal management. The intermittent units have resistances of one quarter that of their continuous counterparts and a power consumption (and dissipation requirement) of four times that of the continuous.

The intermittent solenoid therefore can give a little over three times the force of the continuous units but only for 5 s at a time. 25% duty cycle implies that you'll need to wait 15 s after a 5 s on time before retriggering.