Use readelf -a and objdump -x to inspect ELF files in preference to strings.
Actually, all the GLIBCXX_* versions don’t apply to the entire library, but to each symbol (symbol versioning, see DSO-howto). So you can have e.g: std::char_traits<wchar_t>::eq@@GLIBCXX_3.4.5 and std::ios_base::Init::~Init()@@GLIBCXX_3.4 on the same library file.
The fact that your program needs GLIBCXX_3.4.9 probably means that it has been linked against a symbol that has been introduced/has changed semantics on GLIBCXX_3.4.9.