This because when tribal call to arms is used vassals can decide weaver or not to send their levys to support their liege.
The biggest problem with tribal call to arms is that it stops the AI from being able to asses the relative strength of their own and each others realm as they will only account for the holders levys and not take into account the levys the vassals may supply to their liege.
There are already a couple of mods which do this, but what makes this mod different is that I have tried to properly integrate the tribal call to arms system in a way that it removes some of big problems with the system.
This mod makes all government types use the tribal call to arms method for gaining levees of their vassals.