The ever-changing nature of battle is inherently a common rule of the world; if one lacks even a minimal psychological preparedness for this, then Lann himself would deem himself unqualified as a battlefield commander.
Although, in this battle of hunting the dragon, he indeed wasn't taking the role of the battlefield commander, he still planned and responded to various situations based on his understanding and experience.
Nothing can be perfect, but having a mindset of constantly approaching perfection is essential.
After many worlds and many adventures, this belief became a part of Lann.
It's unclear whether the memories and genetic seeds of the Son of the Emperor influenced him, or if he accepted the life philosophy of the Son of the Emperor.
This is something indistinguishable.
And in Lann's response strategy, the 'miss of the Black Arrow' was simply a very ordinary occurrence.
