These updates occur because the package signature changed. Typically, an underlying library was revised and these rebuilt packages point to, and use, the revised library. A package which does not get such an update will still point to the old library.
If it's a system library (/usr/lib), and you've upgraded with bsd.rd either from -release to -release or perhaps from -current to -current, the bsd.rd upgrade script doesn't ever remove old libraries.
If it's a third party library (/usr/local/lib), the pkg_add(1) tool will delete old libraries during an update only if no other package depends upon it. But more often, because there are packages depending on libraries, it won't delete it -- it renames it to a .lib-* package, so there can be multiple versions of the library in /usr/local/lib. These old libraries will be deleted only after you've updated packages so that nothing depends on the old library, and you've issued pkg_delete(1) with the -a option to delete any automatically installed package (such as dependencies) on which nothing now depends.
If you run # pkg_delete -a now, you may see a few .lib-* packages being deleted.
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