If you're selling downloads with low enough customer volume so far, and your sales and marketing and the product itself are good enough that buyers won't get cold feet when it comes time to actually pay you, then that could well be possible that 100% of people who take their cart to checkout also actually complete checkout and arrive at the thank-you page.
It's rather less likely that 100% of people who simply click an Add to Cart button at all would actually proceed to checkout; however, if you're using Buy Now buttons instead of Cart buttons, those actually bypass the cart and take buyers directly to checkout, so that certainly would be 100% correlation between buyers who click a purchase button to buyers who wind up at checkout.
Since we present the buyer's download link on their thank-you page, almost all download buyers will visit the thank-you page one way or another in order to claim their download. The more buyers you get, the more likely you may encounter the occasional odd one out who has their cookie settings configured in such a way that their conversion won't register, or who never visit their thank-you/download page at all (and who probably request a refund in that case).
