It's possible that what you're seeing is due in part to timing differences - for example, all our sales are captured in UTC (sorry we're not clearer about that).
Also, while we know that Leanpub captures all of the sales in our own charts, it's possible that Google Analytics misses some things; it's complex and finicky at the same time, and while it is a really good way to understand trends and where traffic is coming from generally, for precise business data, the business's own data is the source of truth.
It may also be that coupon purchases are not captured with the Google Analytics goals, which we don't think is something we would have the resources to resolve in the short term, but if you check here:
https://leanpub.com/YOUR_BOOK/individual_purchases
you might see coupon purchases on dates where the Google Analytics results are lower than the Leanpub ones.