Pre-orders are only converted to purchased unit counts if the user ignores the option to decline the (24hr) pending auto-purchase.
iTunes will not charge users right away for the item. Instead, they will receive an email confirmation when the item is released. They aren't charged for their purchase until they download, where some may be eager and some let the timer do it for them, meaning the count doesn't necessarily occur same day as release.
Note that while they are not charged for the entire item when pre-ordered, they still might see a charge. If any portion of the item is already available for download, iTunes treats the pre-order like a purchase and charges them for the currently available content, which they then have access to. Music albums are examples of this.
Also keep in mind that users can cancel pre-orders, refunds have their own influence and timing, and again, iTunes doesn't charge until the item is released & downloaded, so technically they haven't purchased anything, meaning there are no units to be counted as purchased.
That makes sense, the 24 Hour pending period was probably why I didn't see the full conversion on launch day. Thanks!