I read a blog by someone who is very enthusiastic about stuff like logistics in war. A lot of fictional battles are utterly absurd, and it gets worse when you consider wars, the timelines and distances between battles and so on. A lot of authors and directors who write pre-modern battle scenes just don't care about any semblance of realism or basic logic, and the limited time (and the troubles of depicting battles with living, breathing actors) you have in visual media makes it even worse.
Check out A Collection of Unmitigated Pedantry.