It is for each number's range, so a 5% chance each number is out of range on the day the poll is released. That chance could rise every day poll ages if people are changing their minds at the last minute.
Polls should probably just give their ranges instead of their median, but they would also get less press coverage for doing that.
How they handle undecides or build their voters databases can be wild too. One pollster famously won't include you in their sample if you didn't vote in a prior election, even if you attest you have already voted in the current election. Nuts!