What causes glass distortion and why is it NOT a defect in hurricane glazing?

Yes you can minimize it at a cost, but how do you measure it?

To achieve the strength needed to withstand cyclic wind pressures especially at larger sizes, heat strengthened glass is required and it will inherently have some level of distortion.

Inspection standards are
ASTM C 1048-04 - Heat Treated Glass Inspection Criteria
"Distortion, —Thermally tempered and heat-strengthened glass is made by heating glass in a furnace, the original flatness of the glass is slightly modified by the heat treatment, causing reflected images to be distorted."

"The inspector shall place the glass in a vertical position. Inspector shall view through the glass at an angle of 90º Lighting should be daylight level (without direct sunlight)."

"Strain pattern, this is considered a normal part of the heat treating process and is not considered a defect."

"Distortion, at this time no industry quality standards exist"

ASTM C 1172-03 - Laminated Glass Inspection Criteria
ASTM C 1376-03 - Coated Vision Glass Inspection Criteria
ASTM C 1036-06 - Flat Glass Inspection Criteria

However this does not mean that all heat strengthened laminated glass is made the same and some fabricators have methods where they are able to reduce the amount of distortion.

I give classes to architects, builders, and owners on luxury hurricane glazing systems from glass aesthetics and energy performance to comprehensive product overviews and details of all hurricane tested products from most manufacturers in the market without biases.