Understanding some movements

[Haplo_Patryn]

Hi. I'm trying to explain in my forum (Puntadelanza) why the movement that one player is trying to order to his cavalry unit doesn't work and why the IA changes the path everytime. The blue arrow is the path the player wants to order to his unit but the game changes it and order it to move taking a different path.

I think the screenshot is clear enough to understand what i'm trying to ask here (left is what the player is ordering and right is what the IA do). Why?

Thank you.