I'm but a layman but browser-side SSL verification is essentially 3rd-party centralized validation of the authenticity of one side of an encryption mechanism - predetermined vendors tell the browser whether a SSL cert is as claimed and an SSL cert is only an encryption key.

This service doesn't care whether a browser-maker thinks its cert is real; they also provide a means to validate that their downloadable cert is as claimed - the cert is valid encryption between you and them, from anyone not you and them, despite whatever errors a browser throws up.