Why does cell cast acrylic vary so much more in its thickness than other acrylic?
Because of the processes involved in manufacturing them. There are many reasons why cell cast acrylic is often the material of choice on many projects, but close tolerances on thickness is not one of them.
The molds used for cell cast acrylic, typically glass, are subject to their own thickness variations, which may effect the thickness of the acrylic. And of course, each sheet comes out of a different mold, which may compound the difference from sheet to sheet. But even more difficult to control than the flatness of the molds, is the balance of pressure inside vs. outside of the mold. The latter is the primary reason for greater variations in thickness. This is why on sheets larger than 2′ x 3′, it is virtually impossible to guarantee tolerances much better than ± 10% of the overall sheet thickness.
Extruded and Continuous Cast acrylics are both produced on automated equipment which force material between either rollers or belts that are separated by fixed distances. Because they are produced continuously, not separately, there is greater uniformity of thickness. At least more so than cell cast sheets.