On line 123 of sincos.h in VDT we see
c = 1.0 - zz * .5 + zz * zz * get_cos_px(zz);
Error can be decreased by evaluating the expression as
c = 1.0 + zz * (-.5 + zz * get_cos_px(zz));
This is the only change present between the two include directories.
Graphing the error of the two versions on the domain
