modules.builtin and /sys/modules

Hi, I just realized that several modules that are in modules.builtin are not in /sys/modules/ ; why is that? Also I had a case where I made a kernel module builtin and now does not appear in modules.builtin or in /sys/modules/ (but it works as expected and the device is available). Why is that? I once read that every module in the system (builtin or not) is in /sys/module, is that wrong? Thanks, -- Richard