Ineperienced/Experienced/ all other descriptions affect levels. Fusing two monsters averages their levels and gives you the monster closest to that level, depending on demon class.
An Isora is level 14
A regular Cu Sith is level 13
an Inexperienced Cu Sith is lower than level 10
It would make sense if an Inexperienced Cu Sith fused with a Kodama would not make that higher level 14, and instead would make the lower level Apsaras.