Vague or ambiguous provisions in a contract will be interpreted against the "scrivener", otherwise known as the party that either drafted the contract or provided the printed form of contract.The underlying reason (the policy) for this rule is:

The party that proposes the contract is the party that is assumed to have a lawyer and must therefore bear the greater burden in court.

All statements are accurate and applicable.

The party that assumed the responsibility for creating the contract should also bear the greater share of responsibility for any problems caused by vague or uncertain terms.

The party that drafts or provides the contract is, by legal definition, the party that makes the offer and must therefore bear the greater burden in court.