Different Results?

Do people experience different test results between public instance of webpagetest.org vs private instance of webpagetest?

You should see differences based on the computer you are using for a private instance, where it is located, the ISP and network conditions but equivalent agents (say, any of the EC2 AMI’s) will be identical between the public and private instances. The public webpagetest.org runs the same code (identical) as what is released for the private instances.