When things like this happen, it's usually because you have sales that are being processed through multiple processors (think Shopify Payments and PayPal). When you view your payouts within Shopify, if you have any payouts coming from a different processor, such as PayPal, those payouts don't appear listed within your Shopify Admin, as they're handled by a different processor. You can actually check your Analytics Reports to see what payment methods are used, like PayPal, and see if that is where the difference your noticing is appearing.