Laser Cut Plexiglass

When lasers are focused, the beam coming out of the lens is, simply put, tapered or slightly curved as you move towards the focal point.  The sides of the beam after the lens slopes in this curve.  The energy distribution across the beam as well is usually a form of Gaussian curve.  

So if I program a cheap laser cutter to drill a hole though a thick sheet, unless it is a very expensive multi-axis cutting head that can compensate for the beam physics, you will find your drilled hole has a slight taper to its walls, and may have a second reverse taper in the bottom third or so of the length. Laser operators refer to this as undesired  trepanning of the hole. 

Likewise as you cut, without some very specialized optics, if you look on the micro scale, you will find lasers never make exactly straight sides on the cut. More expensive machines and collimation optics can compensate for this, but most low cost laser cutters in the wild  leave something to be desired in terms of edge quality. 

Look at it this way, the focus lens produces a cone of laser light. That cone leaves a transform on the hole if fired blind without compensation. 

Bad explanation, but it's 1 AM here and I've had a long day. Poor choice of words on my part.

Steve 

If you want a diagonal on the edge you would tilt the cutting  head. Something that only a few laser cutters can do.

Steve 

I did some checking. Cutting heads that can control the beam angle use two rotating wedge prisms in series to allow the control of the entry angle of the beam into the work.  These are known as Risley Prisms in the industry.
Effectively the same thing as tilting the cutting head relative to the work.
However the prisms are much more controllable. Two motors, two encoders, two sets of bearings and a pair of optical wedges solve the problem.

However not common equipment  in most local  laser job shops..

Steve