Rebels fighting rebels... Bug or feature ?

[Spendius]

So as France I just conquered Spain and moved my conquering armies back to Paris to prepare my invasion of Turkey.
Soon enough spanish rebels pop up and take Madrid before I can react: Spanish faction is re-established. By the time my repression force reaches Madrid (and after a lost auto resolved battle), the spaniards have also taken back Portugal and start lurking small armies in France and small fleet on my trade routes.
I decide to wait a bit to secure Madrid before taking back Lisbon, but after a couple of turn a second Spanish revolt occurs in my reconquered Spain. To my surprise, the first spanish rebel army moves directly on the newly spawned rebel force and takes them out instead of merging ! These 2 armies had the same spanish flag, yet counted as separate factions ?
Bug or feature ?

[The_Dude]

feature
not knowing the exact political situations one group might have been royalist rebels while the others were republicans.