I'm not really sure if it because Yamakan since Itaru Hinoue also make a special picture for the third vol. of the manga
if you want to know, till vol.5 Kannagi manga always have some special illustration from different artist. for the list:
vol.1: Takeshi Takeuchi (Type-Moon, also illustrated for ep.2 of the anime)
vol.2: Ryukishi 07 (07th ekspansion, the creator of Higurashi and Umineko no Naku Koroni)
vol.3: Itaru Hinoue (Key, also illustrated for ep.4)
Vol.4: Kouga Yun (Gundam OO, Loveless)
Vol.5: Ume Aoki (Hidamari Sketch)