The answer is: a dice. An explanation:
“It’s always 1 to 6”: the numbers on the faces of the dice,
“It’s always 15 to 20”: the sum of the exposed faces when the dice comes to rest after being thrown.
“It’s always 5”: the number of exposed faces when the dice is at rest,
“But it’s never 21”: the sum of the exposed faces is never 21 when the dice