We are using the cividesk sparkpost extension on civicrm 5.5.3 and wordpress 4.9.8. In the last few days, we have sent 2 emails to the full list which is about 6,000 emails each, so it should be showing around 12,000. Civimail shows almost all of the mail is successfully delivered. However sparkpost's summary report for the last 7 days shows 167 accepted, 12 bounced, 0 rejected, 256 rendered, and 179 targeted. What do those numbers mean?
Found the answer, thanks @MatthewWire for the hint. Emails were going out using the "backup mailer" defined at civicrm/admin/setting/smtp because we had the wrong API key installed for sparkpost.