Did a pinch of digging and this Stack Overflow Question seems to be the best "all in one" answer to your question.

To try to explain from what I'm seeing:

• Disable stops any "autostart" services/processes from starting. It also tries to gracefully stop any background processes and services.
• Force Stop kills those processes forcefully (but doesn't necessarily stop them from auto starting again).
• Its possible (though unlikely) for you to Disable an app and there to be a delay for all processes and services to die.
• Thus it can be useful to Disable, then Force Stop if you really want something dead.

That said, in most cases it looks like its more likely to be a "UI Bug". Theoretically, if no processes are running for an app, the Force Stop button is supposed to be deactivated (greyed out), but this check is apparently not very authoritatively checked, and defaults to Active if the OS isn't sure.

Hopefully any wrong info I've stated will summon a true expert to explain better.