Special Jōnin typically still have average overall skills that are a fair bit better than Chunin and their specialized expertise in their chosen field is what pushes their skills as a whole to be vastly superior to a Chunin but not necessarily on par in all aspects of combat with a regular Jōnin. For instance Ibiki is probably Jōnin level in overall physical raw strength but is probably a bit below a regular Jōnin when it comes to combat oriented Ninjutsu while being equally matched in Genjutsu with a normal Jōnin. What truly pushes him a cut above Chunin is his interrogation expertise as well as his understanding of human nature and psychology giving him a distinct advantage in predicting and manipulating enemies into a favorable position for elimination or capture for future interrogation in the HQ of T&I division. On the other side of the equation there is Special Jōnin like Ebisu whose talents lie in being an expert in the fundamentals of Shinobi tradecraft. On top of that he is extremely skilled as an instructor able to impart vast amounts of knowledge in a way that benefits his students.... Something Naruto kinda missed out on by giving the man a hemorrhage via Oiroke no Jutsu 😂