One significant difference a self signed certificate has no CA Intermediate Certificates nor the Root CA Certificates (i.e. a full Testing Chain of Trust)
Root Certificates
The staging environment has two active root certificates which are not present in browser/client trust stores: “(STAGING) Pretend Pear X1” and “(STAGING) Bogus Brocoli X2”. If you wish to modify a test-only client to trust the staging environment for testing purposes you can do so by adding the "(STAGING) Pretend Pear X1" and/or "(STAGING) Bogus Broccoli X2" certificate to your testing trust store. You can find all of our staging certificates here. Important: Do not add the staging root or intermediate to a trust store that you use for ordinary browsing or other activities, since they are not audited or held to the same standards as our production roots, and so are not safe to use for anything other than testing.