People don't understand just because you disbelieve one claim, it doesn't mean you believe the opposite.
E.g. The claim: the coin will land on tails, do you believe or disbelieve this claim?
A lot of people would say "I don't know" because they think by disbelieving the claim they believe the opposite (the coin will land on tails). They don't realise you can disbelieve both claims. This doesn't apply just to theists, many people who claim to be agnostics and not atheists make the same mistake.
E.g. The claim: the coin will land on tails, do you believe or disbelieve this claim?
A lot of people would say "I don't know" because they think by disbelieving the claim they believe the opposite (the coin will land on tails). They don't realise you can disbelieve both claims. This doesn't apply just to theists, many people who claim to be agnostics and not atheists make the same mistake.