I have this question as well. My report sometimes shows fewer Kindle orders than sales, and I would think that all Kindle orders would be counted as shipped as soon as they are paid for, in the absence of pre-orders, which I don't have.
Can someone explain why the numbers are different?
Short answer: It's September (aka KDP Meltdown Month).
Longer answer: Check the marketplace where you're seeing these discrepancies. If they're from India, it's because the entire Indian reporting system suffered serious problems this month. Inquiries to Support are being brushed off, but they're slowly working behind the scenes and reports are being restored to normal. They may not have fixed everyone's accounts as yet.
If they're not from India, use the Contact Us link at the bottom right. However, your inquiry will probably be sent off to the "technical team" and you may never get a reply.