Well, it’s not such obvious tactics, because outside of some of the best premodern armies ever (the Byzantines, the Mongols), people usually went for the complete encirclement. But there are other things that could have happened:
- Gregor charging in recklessly is quite in character – his wing of the battle takes the heaviest casualties in the Battle of the Green Fork, he loses half his men at the Battle of the Fords, etc.
- It could be that Robett Glover is good enough of a commander that he reacted well enough to a bad situation to make the Lannisters pay dearly for their victory.
- Or something else.

