Your findings could be explained by the variation of the wafer's surface
orientation. For example, ef the wafer's surface is misoriented 1 degree
toward the <110> direction, the angles of the (111) planes of the
V-groove perpendicular to that <110> direction will be 53.7 and 55.7
degree with the surface. Check with the supplier about the tolerances of
your wafers.

Alexander Hoelke
Graduate Student

University of Cincinnati
ECE - CMSM