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What's happening is that, sometime during kernel 2.5 development, the memory management subsystem was changed to no longer track data pages shared between multiple processes through copy-on-write, as 'shared'. Instead, only text pages, and pages loaded from shared libraries, are considered part of the 'shared' section, in contradiction to what the top(1) manual page states.

This is generally observable in all 2.6.x kernels and Red Hat's patched versions of the 2.4.1x and 2.4.2x kernels. Reportedly, RHEL 2.1 ES and AS kernels are the same, too.

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If you want a reliable idea of your memory sharing, so far the onlyone way to determine this seems to be to downgrade the kernel to vanilla 2.4.\[12\]\* or test on a non-Linux platform.  There's also patches that can be applied to 2.6 to get useful figures via the /proc/$$/smaps interface; \[http://lkml.org/lkml/2005/7/6/250 lkml thread on the topic\].