Deliberate mistake: Leaven spends too much time thinking about if 645 and 372 are prime numbers to be believable. But she knows the factors of 649 and the result of 26 to the power of 3 almost immediately. Plus anyone with any maths knowledge whatsoever would know that any even number (or one divisible by 5) can't be prime, so 645 and 372 could be dismissed without even thinking about it.
Trivia: All the characters in the movie are named after famous prisons around the world - Holloway, Wren, Leavenworth, San Quentin, etc. The characters match the prison types too - Alderson never meets the group, and isolation is the major form of punishment at Alderson Prison in West Virginia. Dr. Hollaway is a female doctor, and Holloway Prison is a women's prison in England, Kazan is autistic, and Kazan Prison is Russia is a "disorganized" prison, etc.