Would suggest ensuring you're on the latest release, an unmodified theme, and do not have any third party add-ons related to polls.
It is also worthwhile knowing when this topic/poll was created. There was a bug quite some time ago with something similar when it was possible to edit the number of individuals who voted. This capability has since been removed.
If after doing the above, this is still present and it is not an old topic/poll, we would need the topic URL.