User blog comment:G0LD3NF1RE/The Elder Scrolls at E3 2012/@comment-3428817-20120607092634/@comment-5060344-20120607102041

The very first crossbows had latch or crank and so the user's own strength would still apply when using the weapon. That said, the crossbows in Dawnguard do appear to be on a crank so overdraw does not make sense. Lets just put it down to video game magic.