/usr/lib is in /etc/ld.so.conf. This is uneccisary, since ldconfig searches it by default. It makes ldconfig -v complain. Furthermore, it seems to slow down ldconfig somewhat.
/usr/lib is inserted into ld.so.conf by some packages in their %post scripts (because if those packages are relocated somewhere else, they need to register their library paths with ldconfig). Those scripts could test if the library path is different than /lib and /usr/lib, but as it is harmless, I don't think they need to be changed.