The CMake install command's DESTINATION option is sometimes passed a relative directory argument such as "lib" but sometimes is passed the absolute path "${CMAKE_INSTALL_PREFIX}/lib".

The problem is that if an install path is set absolutely at configure time, then it is not overridden by using --prefix at install time. Which makes sense, after all. So when someone does use --prefix foo, some files go to foo, but some files go to whatever was set at configure time (often /usr/local by default).