The word some keeps it vague. When you have some time to kill, you've got an unspecified amount of time — it could be five minutes or five hours.
The word some sounds like sum which means "a total quantity or amount." However, some, unlike sum, refers to an unspecified amount. If you have some money in your pocket, who knows the sum of that amount? Some can also refer to an unknown person or thing. If a person you don't know shows up, you might refer to him as "some guy."