Costa's group goes through the petitions to weed out obviously invalid signatures. Because there are some paid signature-gatherers who are paid only for valid signatures, those signatures are compared to county voter registration records. That takes some time, too, but the signatures submitted to the counties are then much more likely to be valid.
When the report from Secretary of State Shelley came out last month and showed fewer than 100,000 were turned into the counties, even though the dumpdavis.net count was much higher, we were all upset that Costa was slowing down the process. He has since improved, but there is definitely a backlog. The next report comes out on Monday, June 23, for the period ending either yesterday or last Friday (5 business days before the report).
My opinion: Next month he ought to give up on verifying the signatures and just send everything to the counties immediately. I'm not sure when the cutoff date is for the July report, but it's best to ensure the maximum possible number for that report and to force the counties to begin to verify the signatures. Otherwise, Shelley may have more delay tactics available.