What has always gnawed at me

Dredd and Fergie are standing outside of Mega City, looking at the wall.
Dredd explains about the fireball situation.
We can see the fireballs coming OUT of the wall.

They run up the pipe outside to inside, so they should be running towards the fireballs. But they don't They run away from the fireball as if the fireball changed direction suddenly.

Or am I analyzing that scene WAY wrong? :)