You know what? It's actually kind of tough to maintain this whole 'totally straight line' business when you're also keeping an eye out for escape routes. If two kids were to start drifting, say, toward a nice patch of greenery with such an appealing looking tree, it would be pretty hard to actually tell.

Kai's lead

is followed without question

and, lo, they reach the tree. And, wouldn't you know it, along the way, it appears to have caught Shinji's attention, too. Imagine that...!