There are a few reasons for this:
1) If you receive a 3 day ban at a minute to midnight on day 0, then your ban will be up no sooner than a minute to midnight on day 3 (not when you wake up on day 3, not 72 hours from when you made the post that got you banned).
2) A job runs at certain intervals (an hour, 2, 6?) that does a lot of housekeeping, including un-banning. So, in the case above, if the job runs at 6 hour intervals and that interval passed at an hour to midnight on day 3, then that ban will not expire until 5 hours into day 4.
3) You have an expired email address and didn't get the notice of the exact timing of your ban, so, you are guessing wrong as to when you will be unbanned.
No one has ever been manually 'unbanned' for the reason, "my ban is over but I'm still locked out", nor has anyone's Ban timer ever stopped or broken. Will people who are banned ever read this? Maybe. Will staff ever respond to this concern? It has been addressed once here, but probably not again.
The point is, post in accordance with The Rules, and you won't get banned.