I am in DigitalOcean (San Francisco). The answer is simply distance/routing. I am guessing OpenDNS has an anycast node in DigialOcean (SF), where Google Public DNS does not. A ping tells the story.
From DigitalOcean in SF I'm hitting the OpenDNS Palo Alto datacenter. Google is going to San Jose.
Remember OpenDNS is a real business working to provide a service to real paying customers, Google DNS is just a fun little project of building an OpenDNS clone.