Building with CMake-gui - flags overridden

Hi,

I was trying to build GVDB 1.1 for VS2015, Windows 7 professional, using the Cmake gui and some flags are set OFF at the begining of the CMakeLists.txt
for example, this line:
set ( USE_OPENVDB OFF CACHE BOOL “Use OpenVDB” FORCE)
overrides the checkbox in the GUI.

After commenting the line out, everything works, but I would like to know if this was done on purpose to avoid some problem.

thanks
Vladimir