CMake uses the empty build type, which Qt Creator baptized as "Default", to define the common set of compiler parameters for all build types (Debug, Release, MinSizeRel, RelWithDebInfo).
CMAKE_C_FLAGS and CMAKE_CXX_FLAGS will be used for the empty build, which will be generated from the CMAKE_C_FLAGS_INIT and CMAKE_CXX_FLAGS_INIT variables plus the content of the environment variables CFLAGS and CXXFLAGS.
GCC users will get just -O0 build, since that's the default optimization value. It makes no sense to have "Debug" and "Default" also as Debug build.
QMake offers "Debug", "Release", and "Profile".
CMake has "Debug", "Release", "RelWithDebInfo" which works exactly as "Profile", and "MinSizeRel" which could be useful in some cases.
Please remove the "Default" configuration, which just creates confusion, and should not be the "default".