Variance is normal for all acrylic, even within the same sheet. I had this saved in my flies for deference.
Plastic sheet thicknesses can vary by + or - 10% (not only from sheet to sheet, but even within the same sheet).
Nominal acrylic or ABS thickness, actual thickness
1/16", 1.5 mm (~ 0.06") + or - 10%
1/8", 3 mm (~ 0.118") + or - 10%
3/16", 4.5 mm (~ 0.177") + or - 10%
1/4", 6 mm (~ 0.236") + or - 10%
3/8", 9 mm (~ 0.354") + or - 10%
1/2", 12 mm (~ 0.472") + or - 10%