On Sat, 19 Nov 2005 18:20:54 +0000, Rob Horton
wrote:

A distant memory from the
optics section of my engineering degree suggests it's to do with the
"mode" of operation of the laser.

You're nowhere near that limit. The limit in operation here is more
about the lens diameter and the near/far field transition.

Besides which, the better levels have a nice wide line for better
visibility and for better range (it's that near/far limit again).
However the edges of this line are sharp, parallel and well defined.
Although the illuminated area is wide, then useful accuracy is better
than this.