Let's say you have:

    OpenSSL:
      foo1():
        bar()

      foo2():
        bar()

    LibreSSL:
      foo1():
        bar()

Then someone realizes foo2 needs to call baz:

        foo2():
          bar()
     +    baz()

This isn't relevant to LibreSSL and so isn't ported: no problem.

Then someone else realizes that the foo2 change should also apply to foo1:

        foo1():
          bar()
     +    baz()

While this patch will apply cleanly to LibreSSL, the idea that after calling bar() you should call baz() is something they figured out in OpenSSL, and is plausibly a derivative work of OpenSSL.

The more copy-and-pasty the change is, the easier it is to be in a situation where a patch that would apply cleanly to pre-fork code would actually be dependent on post-fork code.